diff --git a/frontends/main/src/app-pages/DashboardPage/Analytics/EngagementTrendChart.tsx b/frontends/main/src/app-pages/DashboardPage/Analytics/EngagementTrendChart.tsx
index 647df5d51d..498b99d657 100644
--- a/frontends/main/src/app-pages/DashboardPage/Analytics/EngagementTrendChart.tsx
+++ b/frontends/main/src/app-pages/DashboardPage/Analytics/EngagementTrendChart.tsx
@@ -2,7 +2,8 @@
 
 import React from "react"
 import { LineChart } from "@mui/x-charts/LineChart"
-import { Skeleton, styled, useTheme } from "ol-components"
+import { RiInformationLine } from "@remixicon/react"
+import { Skeleton, styled, Tooltip, useTheme } from "ol-components"
 import type { MonthlyEngagementTrend } from "api/analytics-hooks/organizations"
 import {
   EmptyTableMessage,
@@ -66,6 +67,34 @@ const TableWrapper = styled.div(({ theme }) => ({
   borderTop: `1px solid ${theme.custom.colors.lightGray2}`,
 }))
 
+const InfoTrigger = styled.span(({ theme }) => ({
+  display: "inline-flex",
+  alignItems: "center",
+  verticalAlign: "middle",
+  marginLeft: "4px",
+  color: theme.custom.colors.silverGrayDark,
+  cursor: "help",
+  "& svg": { width: "16px", height: "16px", marginLeft: "4px" },
+}))
+
+/**
+ * The desktop header housing the same trigger is `display: none` below `md`
+ * (`TableHeaderRow`), so this repeats it once, outside the table, in the one
+ * layout where that header is hidden — never per row, which would turn one
+ * definition into a repeated focus stop for every month.
+ */
+const MobileColumnHelp = styled.div(({ theme }) => ({
+  display: "none",
+  [theme.breakpoints.down("md")]: {
+    display: "flex",
+    flexWrap: "wrap",
+    gap: "16px",
+    marginBottom: "12px",
+    ...theme.typography.subtitle2,
+    color: theme.custom.colors.black,
+  },
+}))
+
 const CHART_HEIGHT = 320
 
 const COLUMN_FLEX = {
@@ -82,24 +111,40 @@ const SERIES = [
     column: "active",
     label: "Active learners",
     color: CATEGORICAL[0],
+    /**
+     * Copied from the `monthly_active_learners` field description in
+     * ol-analytics-api's b2b_dashboard models (mitodl/ol-analytics-api#57),
+     * itself derived from the backing dbt SQL in ol-data-platform. That
+     * description lives only in the OpenAPI schema (/openapi.json, /docs) —
+     * the actual row data this component fetches never carries it, and this
+     * client is hand-written rather than generated from the schema (see
+     * the header comment in analytics/types.ts), so there is no fetch-and-
+     * parse step that could keep this in sync automatically. If the backend
+     * description changes, this string has to be updated by hand to match.
+     */
+    description:
+      "Learners who did anything in a course this month: watched a video, attempted a problem, posted in a discussion, used the chatbot, moved through course pages or earned a certificate. Enrolling alone doesn't count. If too few learners were active, the whole month is withheld to avoid identifying them.",
   },
   {
     key: "new_enrollments",
     column: "enrollments",
     label: "New enrollments",
     color: CATEGORICAL[1],
+    description: undefined,
   },
   {
     key: "certificates_earned",
     column: "certificates",
     label: "Certificates earned",
     color: CATEGORICAL[2],
+    description: undefined,
   },
 ] as const satisfies ReadonlyArray<{
   key: keyof MonthlyEngagementTrend
   column: keyof typeof COLUMN_FLEX
   label: string
   color: string
+  description?: string
 }>
 
 const EngagementTrendChart: React.FC<{
@@ -152,7 +197,16 @@ const EngagementTrendChart: React.FC<{
       <div aria-hidden>
         <LineChart
           height={CHART_HEIGHT}
-          margin={{ left: 8, right: 8, top: 8, bottom: 0 }}
+          /**
+           * `right: 20` works around a `shortenLabels` quirk in
+           * `@mui/x-charts`: for a point-scale axis, a tick's max label width
+           * is `2 * min(space to its left, space to its right)`. The last
+           * tick sits exactly at the drawing area's right edge, so that
+           * budget collapses to `2 * margin.right`. At the default 8, every
+           * month abbreviation gets ellipsized down to nothing; 20 leaves
+           * room for "Feb"/"Dec" to render in full.
+           */
+          margin={{ left: 8, right: 20, top: 8, bottom: 0 }}
           // Horizontal rules only: vertical ones would fight the marks for
           // attention without helping anyone read a monthly value.
           grid={{ horizontal: true }}
@@ -201,6 +255,20 @@ const EngagementTrendChart: React.FC<{
         />
       </div>
       <TableWrapper>
+        <MobileColumnHelp>
+          {SERIES.filter((series) => series.description).map((series) => (
+            <Tooltip key={series.key} title={series.description}>
+              {/* eslint-disable-next-line styled-components-a11y/no-noninteractive-tabindex */}
+              <InfoTrigger
+                aria-label={`${series.label}: ${series.description}`}
+                tabIndex={0}
+              >
+                {series.label}
+                <RiInformationLine aria-hidden="true" />
+              </InfoTrigger>
+            </Tooltip>
+          ))}
+        </MobileColumnHelp>
         <div role="table" aria-label="Monthly engagement">
           <div role="rowgroup">
             <TableHeaderRow role="row">
@@ -215,6 +283,14 @@ const EngagementTrendChart: React.FC<{
                   $numeric
                 >
                   {series.label}
+                  {series.description ? (
+                    <Tooltip title={series.description}>
+                      {/* eslint-disable-next-line styled-components-a11y/no-noninteractive-tabindex */}
+                      <InfoTrigger aria-label={series.description} tabIndex={0}>
+                        <RiInformationLine aria-hidden="true" />
+                      </InfoTrigger>
+                    </Tooltip>
+                  ) : null}
                 </TableHeaderCell>
               ))}
             </TableHeaderRow>
diff --git a/frontends/main/src/app-pages/DashboardPage/Analytics/charts.test.tsx b/frontends/main/src/app-pages/DashboardPage/Analytics/charts.test.tsx
index a394a42376..d315010527 100644
--- a/frontends/main/src/app-pages/DashboardPage/Analytics/charts.test.tsx
+++ b/frontends/main/src/app-pages/DashboardPage/Analytics/charts.test.tsx
@@ -1,5 +1,11 @@
 import React from "react"
-import { render, screen, within } from "@testing-library/react"
+import {
+  render,
+  screen,
+  waitForElementToBeRemoved,
+  within,
+} from "@testing-library/react"
+import userEvent from "@testing-library/user-event"
 import { ThemeProvider, useTheme } from "ol-components"
 import type { Theme } from "ol-components"
 import { factories } from "api/analytics-test-utils"
@@ -144,6 +150,70 @@ describe("EngagementTrendChart", () => {
     ).toBeInTheDocument()
   })
 
+  /**
+   * "Active learners" alone doesn't say what counts as active — the hover
+   * icon's accessible label carries the definition for keyboard and screen
+   * reader users, not just pointer hover. Scoped to the table: `SERIES` with
+   * the trigger's description also drives a second, mobile-only copy of this
+   * same trigger outside the table (see the test below), so an unscoped query
+   * would match both.
+   */
+  test("explains what counts as an active learner from the column header", () => {
+    renderWithTheme(<EngagementTrendChart rows={months} isLoading={false} />)
+
+    const table = screen.getByRole("table", { name: "Monthly engagement" })
+    expect(
+      within(table).getByLabelText(
+        /Learners who did anything in a course this month/,
+      ),
+    ).toBeInTheDocument()
+  })
+
+  /**
+   * The column header carrying that trigger is hidden below the `md`
+   * breakpoint (`TableHeaderRow`), so mobile/tablet users need an equivalent
+   * — once, outside the table, not repeated for every month's row.
+   */
+  test("repeats the active-learner definition once for mobile, outside the table", () => {
+    renderWithTheme(<EngagementTrendChart rows={months} isLoading={false} />)
+
+    const table = screen.getByRole("table", { name: "Monthly engagement" })
+    const triggers = screen.getAllByLabelText(
+      /Learners who did anything in a course this month/,
+    )
+    expect(triggers).toHaveLength(2)
+    expect(triggers.some((trigger) => !table.contains(trigger))).toBe(true)
+  })
+
+  /**
+   * A static aria-label proves nothing about whether the `Tooltip` itself
+   * works — this test would still pass if `Tooltip` were deleted entirely.
+   * Driving real hover and asserting the rendered popper text catches that;
+   * keyboard focus isn't asserted here because MUI only opens on focus when
+   * `:focus-visible` matches (`isFocusVisible`), which this test environment
+   * doesn't set for a programmatic `.focus()` call — see the same caveat in
+   * ContractAdminPage.test.tsx's tooltip test.
+   */
+  test("shows the active-learner definition on hover", async () => {
+    const user = userEvent.setup()
+    renderWithTheme(<EngagementTrendChart rows={months} isLoading={false} />)
+
+    const table = screen.getByRole("table", { name: "Monthly engagement" })
+    const trigger = within(table).getByLabelText(
+      /Learners who did anything in a course this month/,
+    )
+
+    await user.hover(trigger)
+    expect(
+      await screen.findByRole("tooltip", {
+        name: /Learners who did anything in a course this month/,
+      }),
+    ).toBeInTheDocument()
+
+    await user.unhover(trigger)
+    await waitForElementToBeRemoved(() => screen.queryByRole("tooltip"))
+  })
+
   test("shows an empty state rather than an empty chart", () => {
     renderWithTheme(<EngagementTrendChart rows={[]} isLoading={false} />)
     expect(