This is a "skip link"! It's used to make websites more accessible by allowing users navigating without a mouse to quickly reach the main content of the page. Skip links are usually hidden unless you hit tab after first viewing a page. For example, check out webaim.org's documentation on skip links, and press tab key when you arrive at their page to see their skip link implementation.
I've left mine visible because I want to make people more aware of accessibility tools and guidelines so that they can apply that to their own web design work if they haven't already!
For more accessibility info, webaim.org is a great resource, and I also have accessibility tips on my site on the resources page! :)