What does Revelation 12:9 mean?

Some teachers believe the expulsion of Satan from heaven, described here and in earlier verses, refers to his initial rebellion against God before the dawn of human history (Ezekiel 28:14–17). However, prior to this yet-future moment, the Devil still has access to God's throne, where he accuses believers (Job 1:9–11). In the instance described here, Satan and his followers are thrown down to the earth, so he no longer has access to heaven.

In Revelation 12:9 the apostle John refers to Satan as "that ancient serpent," a reference to his appearance in the garden of Eden to tempt our first parents. John also calls him Diabolos in Greek, in English "the Devil," from a term literally meaning "slanderer" or "opponent." Satanas is the Greek word derived from the Hebrew name for the leader of the evil spirits, Sā'tān. This entity is "the deceiver of the whole world," as Jesus said the Devil was "a liar and the father of lies" (John 8:44). Proof that Satan deceives the whole world may be found in those who think success brings true happiness or sex brings lasting pleasure or money offers security. It may also be found in the false religions and cults that reject or twist the truth.