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.
"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.
Scenario: Seth brings his surfboard to the beach on a day when the ocean is evidently very flat.
Surfer: "Wow... I wish I would've brought my long board today."
Seth: "I know, man. I checked the surf report this morning, and there were supposed to be some pretty gnarly waves."
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"
Story Teller: aw man I went to work the other day and this lady came in with her kid... blah blah blah
You: Cool story bro, hope they make a movie about it!
douche: "yeah i was gonna go out with my girl last night but i was really tired from work so i just stayed home and played a round of black ops. but we chilled after and she-
me (turning around to the kids behind me in class); "true story bro?"
Person A: I just got 15 headshots in a row when spraying with an M249 in Counter Strike Source without hacks!
Person B: Got a replay for this?
Person A: No, but I swear it happened!
Person B: cool story bro.