In plain language
Every page needs a title that says what the page is.
Who it affects
Screen-reader users who hear the title first, and anyone managing many tabs.
How to check
Check the browser tab on every page/view — including after in-app navigation: is the title unique and descriptive?. Worked example — A project-management tool's task board: check titles on search results, multi-step forms, or status pages: if the title doesn't reflect the current state ('Checkout' shown identically on all 5 steps), it fails.
Common misconception
A branded title like “Acme Corp” on every page technically has text in the title tag, but it fails this rule because it doesn't identify what the specific page is about. Real example: on a calendar app's event-creation form, the page title in the browser tab doesn't say what the page is ('Untitled', same title everywhere) — users can't tell tabs or pages apart.