Most of the time it's pretty meaningless actually. Lots of forums have the function, but most don't really use it enough for the number to pull much weight or have any significant meaning. This forum doesn't really rely on the "reputation" system even though it has the system in place.
The idea behind it (on all forums) is that users click the add rep button and that user gets a point. The more points someone has, the more you know that they 1) have been around for a while 2) contribute good enough posts that people want to rep them 3) are overall trustworthy forum members. You rep people for useful threads. You can kind of think of it like "liking" or "thumbing up" a video on YouTube. If you see a thread or a post by someone that was helpful, you +rep them.
You can't "rep" yourself, so you can't just spam your points up. Even if you could, like I said, this forum doesn't really use the system so it would be completely meaningless to see someone with a billion reputation points.
Edit: As far as I can tell, you can't even see the rep of other people here, which basically makes the whole system pointless anyways. No matter what forum, I always just go by post count to gauge contribution levels.
