I have been musing on priesthood keys. For now, this is the best definition I have
keys – the ability to unlock, and let someone in, as well as the ability to shut the door, and lock someone back out
keys of God’s kingdom – ability to let someone into the various parts of it. It is laid out like the temple, with an outer court, and inner court, and a holy of holies.
He who holds the keys of the kingdom of God can let people into the the outer court of that kingdom, or into the inner court of the kingdom, or into the holy of holies.