"But what are the keys?"

The "keys" is a metaphor for the ability to unlock (and lock) the "door to the kingdom", which is accomplished through anointed preaching and teaching.

The scribes and Pharisees took away the "key of knowledge" from people via their false doctrines.

The key(s) therefore is a metaphor for that correct knowledge which allows one entrance into the kingdom of God as a citizen, with all its attendant priviledges and reponsibilities.