It is a word commonly used to refer to a friend mostly towards a male, or an expression towards an illogical or horrendous situation, question, sentence, etc., it's use started around the early 20th century originating from the English word brother, used often by African American's slowly turning into 'brer' and 'brah' and now as 'bruh'.
E.g.,
Tommy: My bicycle was stolen so I had to take the subway and arrived late for class.
Charlie: Bruh.
When one of your bruhs is doing you dirty in a game without giving you too much shit talk but isn't holding back on you just because your friends. Could be used as a passive aggressive term when one friend is talking shit but another bruh who's better puts them in their place.
"We were playing Blazblue last night and Dias Bruh Dogged ForeverKnight all night."
"All Dias does is ledge guard in Smash." "Yeah man, hes just Bruh Doggin us every stock"
When you say it once in a server and it becomes a joke in that server.
John: “Yea I’ll play cod tonight”
Elijah: “Man Bruh”
Bruh moment but make it awkward.
Nicole: "Anyssa.... we're the only juniors at this sophomore kickback!"
Anyssa: "Erm *awkward pause* awkward bruh moment."
When someone just says BRUHH in every situation
Person 1: "I got a pet"
Person with Bruh-itis: "BRUH"