You get reputation points from other users when you make good/useful posts or whenever someone feels you deserve rep. You can give other users rep by clicking on this
icon. You get more boxes the more reputation you get, each box represents a certain number of reputation points (not sure exactly how much). When you get negative reps you lose points and if you get enough negative rep, the box turns red. When you build up a lot of rep points, the boxes start to turn dark green like some of mine are.
Reputation really doesn't mean anything on this forum except who has the most boxes.....lol