If someone posts something irrelevant, Contradictic, Or just plain obvious and you want to neg rep that person, You can bundle it together with a comment and say: Dude... wtf -.- I disagree, That wasn't necessary etc.
Or vise-versa....
Your reputation will either increase or decrease depending on your forumer actions, If you're nice,That green little square bellow your avatar will increase. If you're a total asshet or a noob troll, You'll have a red square instead and eventually a few, And people will completely ignore you. I personally do not care for rep, Bad or good... However if you do want to have a large @ss green line, And i see you posting something i agree to or like, I will positively rep you.