Oh sorry... it's 250 posts or one year on the boards... which ever comes first...
The Green squares are a representation of your 'Reputation' on the boards, and your 'Rep Power' is how many positive or negative reputation points you can give others.
If you come across a post that you thought was beneficial, you click on the little picture of the scales all the way to the right of the poster's name and give a 'yeah' or 'nay' and their reputation will be adjusted...
You also gain reputation points by the number of posts and your time as a member on the boards.