They don't mean anything to be honest. Only premium members can give them and there's no real rules or limitations as to who you can give them to. However, this is how the system works:
If you're a premium member you'll see an "Add Rep" button at the bottom left of every post. If you like that post you can click it and it will give the person "Reputation" points which are reflected by the little chicklets under your name. If you're not a premium member you can receive reputation but not give it.
As I said though. It has no bearing on anything. Some of our highest regarded members have almost none and some folks who don't build much have tonnes. It used to be a bit of a game where people would ask for points and get them "just because". That skewed the system so that it can't be used for any meaningful recognition.