rep (reputation) are those green rectangles under your name and picture. It's almost the same as liking someones post. When you rep someone, they get a certain amount of post depending on how much rep or posts they have. (don't know which one it is) Every 100 rep points you receive, you get 1 more green bar. Up until you get 500 rep points (5 bars) it becomes 200 rep points needed. You rep someone by clicking the red heart on the side of their post.