What is “Skip to main content”?
In any Salesforce page, if no element is in focus and you press Tab⇄, a hidden element Skip to main content is put into focus.
What does Skip to main content do?
The purpose of the "Skip to main content" link (skip link) is to allow users who don't have the motor control to use a pointer device, or who use assistive technologies such as screen readers and can't see the screen (and thus use a keyboard or other device to navigate), to bypass repetitive header content and navigate directly to the main content on the page. If you use the tab key to navigate through the page, you'll see that the skip link appears when it receives focus, and that pressing the return key while it's focused will take you to the main div. If you press the tab key again after that, the next element that receives focus will be in the main content section. If you had not used the skip link, you would have to tab through all the links and tabs in the header area before getting to any useful content.