Reputation is something that represents how popular someone is.
Typically users distribute reputation when they feel that the user is trusted or deserves it after a post/thread.
For it to increase, you click the start button right beside the triangle with the exclamation mark.
For the decreasing part, it has been disabled.
To find more about your reputation and how much reputation you can distribute, check your settings.
Misusing the reputation button can result in to a ban.
"- No misusing/soliciting forum rewards, including on yourself. (However putting along the lines of "Rep/Thank me if I helped you" is acceptable) (3 Day Ban)"