I only introduce a few things at a time. Max 1 page info dump, plus discussion before session zero (you want to play in X campaign or Y) and during session zero. Then, if/when things come up their PCs would know, I let them learn it / tell them. I think, and this is 100% IMO, that it doesn't matter as much as many believe, that the players really KNOW the world and stuff all that much.