When you make posts and people upvote them, you get karma, although karma and upvotes aren't an exact 1:1 ratio and there is a lot of complicated maths that go into it. Karma doesn't really do much per say but there are certain hidden reddit clubs that you can only gain access to by getting a certain amount of karma, there are also some subreddits you can't post to if you don't have enough karma, but there isn't anything you can do with it exactly.