Answer:
1.How do you interpret this quote?
The Pope is simply stating a well-known fact at the time: the Catholic Church had more political, economic and religious power than any single kingdom of the Middle Ages, and the power of kings were derived in great part from the support of the pope.
2.Why do you think Pope Innocent III would say this quote?
The Pope probably said this quote in order to reassert his power. He probably was questioned or challenged by some monarch, and the Pope felt the need to make a statement like that.