This website shows your HTTP referer. As simple as that. Your referer is the page you're coming from. Why would you want to know your referer? In other words: Why should we tell you which website you visited just a second ago?
This service is mainly used by people who want to test whether their anonymization service or their browser settings work correctly. For example, you might want to search for the phrase "what is my referer" on Google and click on the result "whatismyreferer.com" (this page) to see if we recognize Google as your referer.
If you came to this site by clicking on a link on another page and can't see your referer above, that means neither we nor other website owners can track your source. You're one step further on your way to anonymity on the internet!