Every people has their reputation towards each other, if somebody helped you its up to you if you want to give him/her a plus rep! you can add or minus his/her reputation using the + and - buttons at the right part of his post right here
---->you should see a
+ and
- signs right there... thats where you use your repping powers.
1 user is only allowed to use his/her Repping power once a day.

cheers!