A skip link is the first focusable element on a page -- typically <a href="#main-content" class="skip-link">Skip to main content</a> -- styled to be visible only when keyboard-focused. It lets keyboard + screen-reader users bypass the site's primary navigation on every page rather than tabbing through 30+ menu items.
WCAG 2.4.1 (Bypass Blocks, Level A) requires a mechanism to skip blocks of content repeated across pages. Skip links are the simplest implementation; properly-marked-up <main> / <nav> / <aside> landmarks (which screen readers can jump between via the D key) are an alternative + complementary mechanism.
The destination (<main id="main-content" tabindex="-1">) needs tabindex="-1" so the browser can move keyboard focus to it on click; otherwise some browsers move scroll position only, not focus.