see the green dot above your gamertag? that is you reputation, or rep. this basicly shows how much you have contributed to the forum. when someone finds somthing new, or says something funny, or makes something cool, or just does something you like or approve of, you click on the middle button on the bottom left corner of their post where it says "add to so and sos reputation". rep doesnt really do anything, its just kind of an ego boost. notice how you only have one dot? thats cuz your rep is under 100. i have 2, becaus my rep is 121. you get a dot for every 100 points (i assume, i am sorta new so someone else could explane it better). i also belive that the higher someone elses rep is, the higher amount of rep they can give you. for example, i could give you +1 rep, while someone like Projectego or Albionknight could give you +10rep. mmmmkay?
edit: you dont have to say "+rep" buy the way. but if you do, youd better add to there rep! i think mostly people say +rep to let everybody else know that they are not a jurk and added to the persons rep.
also, this post is an example of somthing that deserves to be +reped.