Question mark at the end of URL?

So, when I go into a level, The URL has a question mark at the end. for every level.
But it’s not part of the URL because when you copy/past it in the forum, The question mark is black, while the URL is blue like the link that it is. Does anyone know what’s going on?

It’s called a:

It’s used for adding extra info or a different mode to a URL. Here’s an example where it’s actually used (normally you don’t need it):


wow, you really wrecked yourself.