It is a per-post feature.
You ask a question and someone answers it and you like the answer.. so you 'thank' him/her. or just you show your appreciation to someone who posted something that you found interesting
Also if you take a look on each posters info (to the left

) you will see a scale. This is the reputation icon which allows you to add to or remove from a post's reputation value.
or for a more detailed description of such features take a look at Sharon's guide at
Unique Tips for NoBS Network
ps. i just thanked you so you can see the effect under your post