I will explain what rep is, for those that don't know what it is.
Rep is basically the same thing as what Thanks was on the old forum.
You give rep by pushing this button or In the Dark GF themes.
You do not have to give a reason for the rep, that is just an optional field.
So there you have it, rep explained. So if you find someones response helpful, or someone helps you etc., you can leave them +rep. If you think someone has done something wrong, you can also give them -rep, just like you could do with Thanks on the old forum.
Hope this helped
- Slacker