What do young guys mean when they call eachother bros and say "thats a true bro dude" or something like that?

Well my youngest son Jason who just turned 28 and is very much into rock n roll, skateboarding, long hair, tattoos, bikes, partying and things like that, hes been pretty much a "job floater" since he got out of the Marines back in 2008. Hes always talking to me about things that I have no idea about (I'm 60 and a Vietnam Vet). He tells me "oh thats a true bro song right there" and he claims that the movie "The Hangover" is a "bro movie", what does this mean exactly? I really don't know much about the lingo and I am curious but would rather not ask him, my other 2 sons are in their 30s and have no idea either.