A response to a story or piece of information that you find boring, or is too long and you don't want to read it.
One extra note, though. The only proper use of "Cool story, bro" is to say it exactly as it is in the quotes. Any other variation ("Interesting tale, brethren", "Wonderful retelling, sibling") should never be used under any circumstances, and will make you look like a complete failure to everyone who reads it.
Me: Yeah, so I was at the mall eating some corn dog when this guy walks by.
What a douchebag says when (s)he wants to completely disregard a separate party's important and/or drawn-out speech. Contains a similar connotation to "tl;dr" but with an elevated amount of douchefaggottry.
Guy#1: Alright, look. I brought you here to talk about Jess. She keeps telling me that you've been sending her inappropriate texts and aggressively grabbing her ass in the hallway. I know we're friends, but seriously, the two of us are in a relationship. Back off. Respect both of us and give her some space and I won't have to come at you.
Guy#2: cool story, bro Guy#1: You know what? Fuck you.
A sarcastic way of acting like you give a shit of what the other person has to say. Usually this phrase best pertains to Facebook updates and mundane stories with pretty much no point or purpose.