See that little check mark button thing under your name and info stuff? That's the rep button. In essence, if you help someone out in a post with a suggestion or fix or something, that person can click that button and give you reputation points (also make a comment). Same goes for you. Also, before you rep a person again, you have to rep someone else first. It's a way to show off that you are nice and helpful and have some serious knowledge

p), basically a reward system. You can look at your rep comments (also gives a link to the helpful post you made) in the User CP. When you register, you start off with 10 rep points. Also, admins can remove rep points for doing bad things such as spamming, double posting, abuse, etc