17:10and they are seven kings; five have fallen, one is, the other has not yet come; and when he comes, he must remain a little while.

This is the middle verse of the triplet which explains the seven heads of the beast.

and they are seven KINGS;

Nearly all commentaries try to identify these KINGS as either great empires or great leaders. Many try to name specific empires, but, this is risky in light of the first sentence of the previous verse .

The most that should be said is that seven is the number of completion in Revelation. Therefore, this should be interpreted as being the sum of the "character" of the beast. Furthermore, since the Bible is primarily an "Abraham" book, the Jewish and Arabic viewpoints should be predominant.

five have fallen,

At the time this vision/explanation is happening, five spiritual eras or entities (this verse doesn't distinguish) that have tormented the Jews will have disappeared. While they could have been empires in the distant past of Jewish history, they could also be abstract entities which have spiritual impact. For example, they may be humanism (Genesis 3:1-7), naturism (Romans 1:24-25), atheism (1 John 5:6-10-12), polytheism (1 Corinthians 8:1-6), or materialism (Revelation 18:9-16). After the 7 seal judgments, the 7 trumpet judgments, and the 7 bowl judgments, these KINGS (kingdoms) are likely to be greatly diminished.

We know that the beast (devil) has kingdoms. He tried to give them to Jesus if only Jesus would worship before the devil (Luke 4:5-8). Apparently, at this point in the tribulation, five of these kingdoms, whatever they may be, are done away with.

one is,

Relative to our lifetime, this probably is the Moslem world. However, this also could be hypocrite Christianity (Matthew 7:21-29).

the other has not yet come;

Since this is beyond our relative horizon, we cannot know what this seventh KING (kingdom) is. We do know that the Antichrist is part of it (next verse).

and when he comes, he must remain a little while.

This final attempt at suppressing or exterminating God's chosen people (see verse 14) will only last a short time (as in Revelation 12:12).

What is striking about this phrase is its indefiniteness. Normally, earlier in the Revelation, times are very precise (Revelation 9:5, Revelation 9:15, and Revelation 11:3). Perhaps it is written this way so that the devil will not know how much "little while" he has. He can read or hear scripture, too.