It's a means of rewarding a member for the informative quality and knowledge of their posts.
On the same token, one can be issued negative reputation points if it is felt that they haven't a clue as to what they are talking about or are generally just disruptive to the context of the thread.
This is done by clicking the little scales in the upper right of a members post.
To some, it has become a game and therefore semi-negated the purpose of it.
But what the hell do I know ... I've only got one little green chicklet and a Rep Power of 3.
Or if you think something someone said is funny, you give them a rep point. They all add up and if you don't have enough god won't let you watch TV with him.