Rep power are reputation points that other members can give you (like i did for you

)....you can have 500 post but only 50 reputation points.
Chica heres "rep power" explanation from Freeones FAQ:
Reputation is a way of rating users depending on the quality of their posts. If the administrators have enabled reputation, then the reputation icon will be visible in posts.
Reputation may be positive , negative or neutral . Negative reputation may only be given if the administrator has enabled this. Neutral reputation is only given when the person giving reputation does not meet the criteria set by the administrator to affect someone else's reputation.