QB Coach is the guy going through the film with the quarterback and showing him what he missed, what he should do in that situation next time, asks him what was going through his mind on that play, etc.... Its basically an OC but strickly for the QB, some work on mechanics if they see a weakness in a players game. They run the QB drills in practice, etc.
IDK why people dont understand this. Youve never played peewee football? you got a guy that teaches the basics of a position, runs the drills to work on skills, and goes through their piece in that weeks gameplan.