Coolstorybro is most commonly said by 12 year old faggots.
It used to be used on the image board 4chan as a reply to a long, descriptive post.
But now the phrase has been spread around the internets by stupid fucking kids who are trying to be cool.
If you see someone typing this, flame them immediately.
Jim: Oh, so you're copying off the other kids now..
Jim: Fucking idiot.