Dictionary
Landmark
A region of a page, like nav, main, or footer, that a screen reader can jump straight to.
In more detail
What It Means
A landmark is a named region of a page, created by elements like header, nav, main, and footer, that a screen reader can use to build a quick outline. Without landmarks, a screen reader user has to listen through the entire page from the top just to find the main content.
See it in context on the homepage: Semantic HTML: The Layer Underneath.