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?
1) Chad: I ate an apple for breakfast
Angie: Cool story bro.
2) Hannah: And then I rinsed my teeth. And then, lo and behold, I saw a black mark. I thought: "Hmm, what can it be?" and hark! It was a cavity! So then, I --
Josie: Cool story bro, you should write a book about it.
3) Tad: boring and lengthy story about scientists growing a new type of capsicum
Beth: Cool story bro, in which chapter do you shut the f*ck up?