Revelation 7 showcases two groups of God's people: the 144,000 sealed from the 12 tribes of Israel and an innumerable multitude that has come out of the great tribulation. I have less to say about the former than I do about the latter.
Many theological ships have run aground on the idea of the 144,000, most notably the Jehovah's Witnesses, who used to teach that this number represented the people who would be saved through the Jehovah's Witness movement. When their membership rose past this number, they had to do some major re-interpreting.
But the great multitude! I like the emphasis here on the totality of the world represented. There are members from every nation, tribe, people, and language. This sounds (to me) an awful lot like Matthew 24:14: "And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come."
Some end-times scholars assert that a precondition of the end is that the gospel must first be preached (successfully generating converts) in every nation or "people group" around the world. They believe that there will be a spiritual harvest for God among every facet of humanity, and that this will be to God's glory.
While this interpretation seems to be the case from a plain reading of the Bible, I would caution that it's always risky to insist on our preconceptions. To say, "Jesus Christ CANNOT return until every people group is reached with the gospel, and that is years away!" may leave you with some serious egg on your face if He returns tonight! A larger, more stressed truth in the Bible is that Jesus will return "like a thief in the night" and that it could happen suddenly and without warning. The problem with requiring an "every people group reached" dating of the end-times is that it depends on OUR definition of what a people group is, rather than God's. The Bible teachers that I admire most tell me that everything has been in place for Jesus to return at any time since the first century. We are not necessarily waiting for that last piece of the puzzle to fall into place to "allow" God's timetable to continue.
Finally, one last observation: I really love the imagery of verse 17: "For the Lamb at the center of the throne will be their shepherd; he will lead them to springs of living water. And God will wipe away every tear from their eyes." In this verse we find that the Lamb is also the Shepherd! Both of these images were applied to Jesus in the Gospel of John (the Lamb that takes away the sins of the world and the Good Shepherd.) I also appreciate the reference to springs of "living water," another cue from John's Gospel. I can also see echoes in this verse of the pastoral vision of Psalm 23, being led beside still waters, with a restored soul.
What a great God we have!