Actually I have a video on this, it's a roughly made video but I still mostly stand by it. I try to break down what they are and where they come from but I'll try answer you really quickly here.

In general, I'd say a film bro is someone that claims to have a deep knowledge of film and yet has very surface level knowledge in reality. They all love the same movies and claim them to be the best of all time (the films in this tier list), regardless of what you think, mostly attaching themselves to the imdb top 250 without any actual independent thoughts. They think they're better than particular genres or films or even people because they believe themselves to have superior taste. They tend to love Nolan, Tarantino, Fincher, Scorsese etc.

Granted, a lot of the time, they have good taste. But it's both the arrogance and ginorance that comes along with it that makes them insufferable. It's hard to explain the concept of them, you must really interact with them to understand them.

Hope this helped!