It converts your old chain (usually wider and heavier) to a lighter narrower chain with a different pitch to it. Doesn't last quite as long as, say a 530 chain does, but will quicken your throttle response a little because your motor doesn't have to turn a heavier chain. you would have to buy new sprockets as well too. My bike comes stock with a 525 chain and it's not too much bigger than a 520 so I think I'll keep it.
From waht I've heard it's the thing to do.