Look under your name in this post. (Top left of your post)
Its just a title ("Position") related to how many times you have posted. The idea is that as you post more on the forum you establish more credability and know more so you are rewarded with an improving title. In this case, its higher HP.
The system encourages people to be a part of the community rather then just reading the threads and never adding to the discussion. Sadly, the system also incentivizes some people to post a lot regardless of the quality of their posts.
I would like to thin that the up is more the participation of people and the karma would be more the quality that they bring to the table. I think that all posts are important, even if they have no real meaning.