"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.

A phrase sarcastically used to indicate one's disgust or indifference towards a tl;dr story.

An appropriate response to a story that went nowhere and/or was completely uninteresting.

4chan saying commonly used when sayer is listening to someone talk to him and doesn't really care

Cool story, bro is the ultimate shut down to faggotry, obvious trolling, and tl;dr infecting the internets, to which there is no response

The most suitable response one can give to a story that completely wasted ones precious time. Parents, co-workers, and history classes often provoke its usage.

This term can be used, within the company of those you are close with, to in fact, congratulate a bro on a good story.

A phrase used to express indifference or boredom to a long or boring conversation.

There are many different types of "cool story bro". There is:

1) cool story bro
2) cool story bro, you should write a book about it
3) cool story bro, in which chapter do you shut the f*ck up?

etc.