You can add or deduct from someone's reputation by clicking on the little star that is bottom left of every post. It is anonymous but you are encouraged to leave feedback that is constructive (e.g. if there's a post you don't like then calling someone a jerk or similar is not considered constructive). Receive enough deductions and your green box will turn red which kinda means that mods will be watching you pretty closely....i.e. that's not good. The rule about having to spread reputation around prevents anonymous bullying.
BTW, points can be redeemed from the TMC gift catalog you received in your welcome pack.