People can leave you "rep" or "reputation" for any post you make. You can see what they have left you by clicking on "User CP" in the top left. Rep is about halfway down. I left you a comment so you can see what it looks like. The longer you have been on the board and the more Rep you have been given, the more points your rep will have.
You leave other's rep by clicking on the little white icon in the bottom left of any post. A little will pop up and you can leave them positive rep or negative rep. They are the only one that can see the comments that you make.
Positive rep is usually indicated by a little green light, negative rep has a red light and when someone reps you that hasn't been a member long or doesn't have much rep, there is no light. Just means it's 'neutral'.
Hope that makes sense.