It's points given by other members. If someone gives you a +1, they like your post enough to do it. If someone gives you a -1, most of the time it's usually because it's something that's flatout offensive. If you click on the link "+0/-1" in the post it was given then you can read the message they gave that says why the reputation was given.
If it was a reputation point on you and you don't think there's anything wrong at all and you care enough, you can ask a mod if they want to remove it, and they will if they agree with you.