It's a dismissive statement so if you only have 'cool story bro' as a post then you could get banned for it.
An appropriate response to a story that went nowhere and/or was completely uninteresting."This one time me and my brother were eating sandwiches and he was like 'dude, I hate mustard' and I was like 'yeah.'" "Cool story bro."
Aha.I looked it up on Urban Dictionary, here's what it said: