What does Revelation 9:1 mean?

When the fifth angel blows his trumpet, a star falls from heaven to earth and receives the key to the shaft of the bottomless pit. Clearly, the "star" in question is not an inanimate object, but a person. Isaiah 14:12 refers to Satan's fall from heaven and calls him "Day Star." At some point prior to the dawn of human history, Satan, also referred to as Lucifer, rebelled against God and led other angels in the rebellion. Some of these fallen angels have been locked up in the Abyss, which is another term for the bottomless pit.

However, Satan receives the key to the Abyss and releases them. Some Bible teachers equate this event with the description of Satan's banishment from heaven in the middle of the tribulation. Cast down to the earth, Satan will relentlessly pursue Israel and hope to destroy her (Revelation 12:1–10).

It is important to observe that the key to the Abyss does not belong to Satan. This verse explicitly says that this key must be given to Satan. God permits him to have the key, and to unleash the means of this particular judgment, so sinners will see the end results of their own evil.