Rep doesn't really "do" anything. It is merely a measure of how other users feel about you. This generally holds true: an experienced, well-liked, and active contributer will likely have many green bars. That isn't to say those with the same characteristics must have high rep, but that's just a general guideline.
You click the little scale dealie under their username to add rep. I think that the more rep you have, the more "power" your rep has. So my 10-month-worked-on rep would have more power than a new member.
Peter