@smolbaka boymode has a lot of meanings based on context, the two most common meanings is a crossdressing guy who isnt crossdressing, usually when a crossdressing guy is streaming its awkward to say "ok im going back to being a guy/dressing as a guy" as their online persona is supposed to be "a girl" even if the viewers know otherwise, so they say "boymode" to suggest a girl turning into a boy, rather than a boy turning back into a boy
the second common usage is for trans girls who for whatever reason need to pretend to be a boy, usually occurs when you go to a school or you live in a family that wont let you be a girl openly, or your workplace wont allow it, so you "boymode" rather than "be a boy" since of course, a trans girl wouldnt willingly imply they are a boy, for obvious reasons. there are other contextual meanings but it's pretty rare so not significant. in the trans girl context, boymode is sometimes replaced with "crossdressing", a trans girl dressing like a boy would be "crossdressing" though that word choice is not very common, likey to do with the fact trans people want to stay away from the concept of crossdressing since people confuse the two concepts too often