general Activity will increase your level. Posting, Threading, Getting responded to (footprints), all those will help to increase your level.
As you're level increases your rep bar gains in size. Someone with say a level 7 may have somewhere of a 700point rep "life" where as a more tenured vet who has say a level of 38 has over 5,000 point rep "life".
Also as you increase in levels, you're rep "power" increases also. Again that same level 7 may have 25point rep hit where my level 38 hits around 260.
One of the many systems built into this site for activities that go beyond just posting. We've actually had "Rep Wars" before where the main goal is to just "neg" the opposing team down to Zero... like a big game of tag I guess.
You know, I know all about it, and I'm still not sure what it's for. If you've annoyed people enough to where it stops being green, then you're going to be banned before that gets a chance to happen. So I guess a full rep bar is like a trophy you get just for participating: if you feel special for having it, then you probably are... but not in any way you'd want to be.