Its basically what other forum members think of you.
If you navigate to a forum members profile, you'll see in there information box a section called reputation. It'll have 2 choices, one called details, which will show a list of people who have voted, and their comments. The other is called Vote, and is where you can vote for that person.
I've just gone to your profile and given you a point. Now everywhere on the forum where you've posted, you'll see your reputation point as 1.
If you're familiar with eBay, it's roughly the same as the feedback system, only not as serious.
Oh I see thats cool. Ive bought TONS of stuff on ebay and never pay back my Bro XD