'Key' in the Bible

"To the angel of the assembly in Philadelphia write: "He who is holy, he who is true, he who has the key of David, he who opens and no one can shut, and who shuts and no one opens, says these things:

The fifth angel sounded, and I saw a star from the sky which had fallen to the earth. The key to the pit of the abyss was given to him.

I saw an angel coming down out of heaven, having the key of the abyss and a great chain in his hand.