A back-to-top button moves someone from lower down on a long page back to its beginning. It is like an elevator button that saves a long climb up the stairs.
This can be useful in a long article or list, especially on a phone where scrolling takes more effort. The control often appears after someone has moved down the page.
Give it a name that explains the action, and make sure it does not cover important page content. On short pages, ordinary scrolling may be easier than adding a button that people rarely need.