Bruh when taken on its own means nothing, it is entirely dependant upon the context of a situation for meaning. It's similar to something like an exclamation mark, or using all caps in that way. Using it in a sentence, or even just on its own, typically adds some kind of emotion of confusion or disbelief to a statement. For instance, "what?" can have many meanings based on context, but saying "bruh what?" specifically implies that the cause for confusion was because the original statement itself made no sense.
Random person: Dude, did you check out that new Metroid game?
You: Bruh.
Random person: Hey, did you see that guy throw their water bottle into a microwave?
You: Bruh.