The phrase a person uses when he/she couldn't possibly care less about what you have to say, yet feels the need to satisfy your "story" with a response. Very commonly received when the person using the phrase feels as if they are superior to you in each and every way, and would like nothing more than to pound your face into the ground.

As made famous by Katie Dalmas.

when someone tells you a boring story you don't wanna hear

Not a cool story.
(And not necessarily towards your/a "Bro".)

"Cool story, bro" is an expression used to actualize the insignificance of stories or statements that are falsely believed to be very significant by the individual that is sharing them. Usually a condescending response to a self-indulgent based monologue, this phrase is often used when the speaker's story is, in acutuality, not very cool at all. If used to positive effectiveness, the person(s) applying this expression may sufficiently help the speaker to realize how insignificant their story and/or statement truly is.

Something retards and trolls say, when they didn't like something they read, really want to argue but, just aren't smart enough to do so.

See also TL;DR

you all know the definition to cool story bro, but now there's an addition. If you really don't like the story your friend just told, add on "Hope they make a movie about it"

a troll's last resort, often used when the troll has no other defense to use