contribute something someone likes to the websites and they might
+rep you. if you piss someone off, they might
-rep you.
you can choose to "approve" or "disapprove" of a persons post, and leave a reason why after clicking the
icon on the
top right of their post.
Based on my experience, 100 points is an additional rep bar, but I could be incorrect as I don't really have enough rep bars to judge