In plain language
There must be a way to skip repeated blocks (header/nav) — usually a 'Skip to main content' link.
Who it affects
Keyboard and screen-reader users who otherwise re-traverse repeated headers on every page.
How to check
Press Tab once on page load: a skip link should appear; activate it and confirm focus lands on the main content. Worked example — A news outlet's article page: press Tab on load: if the skip link is positioned after the navigation block it's meant to skip, it fails to serve its purpose.
Common misconception
Adding a skip link satisfies this rule only if it actually works, many implementations point to an id that doesn't exist, or the target isn't focusable, so the link does nothing when activated. Real example: on a ride-hailing app's trip booking screen, the page repeats the same navigation menu and promo banner above the main content on every screen, with no skip link, landmark, or heading that lets a keyboard user jump past it.