: is your commandline, it does a lot of actions. w is write, q is quit. So you're doing a write quit, the ! just tells it to fuck off and do the fucking command essentially.

If you open a read only file, and you need to quit just hit it with the :q! or if you haven't made any changes, :q will work just fine.

naturally, as one would expect, you can hit all of these independently. :w just writes changes, :q will close as aforementioned.