What does Revelation 17:10 mean?

According to this verse, the seven heads of the beast (Revelation 17:3) are seven mountains, and these seven mountains are seven kings. The angel declares that five of them have fallen, one is alive, and the seventh king is yet to come. When he comes, his reign will be brief.

Perhaps this reference is to forms of government which dominated the Roman Empire. If this interpretation is correct, five of the forms of government had already fallen. They were kings, consuls, dictators, decemvirs, and military tribunes. A sixth form of government existed in John's lifetime: the imperial government under the Caesars. A seventh form of government, the Revived Roman Empire, would therefore lay ahead—as of the time John wrote these words—and would exercise control of multitudes of people in the tribulation.

Other interpreters identify the five fallen kings as Augustus, Tiberius, Caligula, Claudius, and Nero. They identify Domitian as the sixth king and the head of the Revived Roman Empire as the seventh. Others tag seven empires as the seven kings: Egypt, Assyria, Babylonia, Persia, Greece, Rome, and the Revived Roman Empire as the seventh. Again, given the symbolic and speculative nature of Revelation, it's important not to be overly dogmatic about any one specific viewpoint on this issue.