Click on your User CP. There, it will show you the last five reputations you received. When someone clicks on the scales at the top right corner of a post, they can say they approve of a post and give the poster positive reputation. (It is also possible to take away reputation, if someone posted something rude or inappropriate.) Somehow, it judges the quality of the post, and gives a certain number of points.
For example, someone could post "I like pillows". Should I give them reputation for that, it might be neutral, which is gray. Or, if someone posted a deep, thought-provoking post, then it might be green, and points would be added.
Hopefully this makes sense...