Rep Power is when you read a post, and you think the person who wrote it was....helpful, supportive or made a valid point and you can 'rep' them.
To rep someone you click on the bottom left of the said post, on the little scales icon. This then gives you chance to write a short comment and add to their reputation as a useful and supportive poster.
Rep comments left for you by others can be viewed if you click on 'User CP' and they are below your subscribed threads.
Hope that helps x