'Key' in the Bible

“And to the angel of the church in Philadelphia write:He who is holy, who is true, who has the key of David, who opens and no one will shut, and who shuts and no one opens, says this:

Then the fifth angel sounded, and I saw a star from heaven which had fallen to the earth; and the key of the bottomless pit was given to him.

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