With the button at the lower left bottom of a post you can give a poster reputation for that post. Reputation given can be negative or positive, depending on what you think of the post. The amount added or substracted from the receiving user's reputation count (displayed under their username) depends on the user rank, post count and registration date of the one giving or taking reputation.

The idea is that people with high reputation generally contribute well to the forums, and probably that it therefore encourages people to post better.