This tried, and true way of play was passed down to us from the ancient dice rollers of old. It goes by many phrases. DM's word. DM's God. Smoke fills the room. Either way, it means that your DM is there not to contest you as an individual but as a player. They are there to keep the internal rules running. The rules, however, are guidelines at the DM's discretion. It depends on the person you have dubbed: the person most likely to consume these books and not call it homework.