Maybe this is a language barrier issue but what does backlog mean on this site, and what is it used for?

It's meant to indicate which games you have in your collection that you intend to finish at some point. Also, there's an account setting to change whether you want things to be backlogged by default or not.

Or alternatively, it could be used to describe the games you intend to at least try at some point. It's up to each individual to define it.

The games in your backlog that you haven't tried/finished are alternatively known as "The Pile of Shame".

Officially a backlog is a pile of work yet to be done. In this case games that you want to play.
The nice thing about a backlog is that is needs a definition of 'done', so it is up to you to decide when a game is 'done'. You can also put it on the backlog again after playing some, or get it off the backlog and archive it when it is no longer valid (out of scope)