Use <nav> for a page's major navigation blocks, the sections whose job is linking around the site or the page:
Main site navigation
A table of contents
Breadcrumbs
Pagination
Not every group of links needs it. Footer links usually don't, unless they are primary navigation.
The element maps to the navigation landmark, so screen readers can jump straight to it or skip past it. A page can have several <nav> elements, and labelling each one with aria-labelledby lets screen reader users tell them apart.
Rewriting in plainer words…
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