...
Using the TAB key allows users to move forward through each interactive element on the page (links, text fields, search, buttons, etc.). Using the SHIFT+TAB keys allows the user to move backward through the same elements. As each element is focused it will be surrounded by an outline, in order to enhance its visibility from other elements on the page. The user can trigger an element's behavior by pressing the ENTER key, e.g. once a link is focused, pressing ENTER will open the link in the browser.
Skip to main content
The first element on each page - when using the TAB key - is a "Skip to main content" link. This link moves the browser focus to the main content of the page, minimizing the number of times a user has to hit the TAB key to get to the most prominent part of the page. When viewing an audio or video page, the "Skip to main content" link moves directly to the Play button.
...