At the heart of chapter 7 is a promise that those who belong to God will belong to God forever. No amount of persecution, tribulation, trials or struggles can separate us from our loving creator. St. Paul, in Romans 8, wrote that "nothing else in all of creation will be able to separate us from the love of God in Christ Jesus our Lord." So, here in 7, we find the remnant sealed from the people of Israel. 144,000 is 12 x 12,000, remember the number 12 here has "symbolic" rather than actual significance. Also note that beginning in verse 9, that beyond the symbolic inclusion of the tribes of Israel is a "multitude that no one could count -- from every nation, from all tribes and peoples and languages, standing before the throne . . . " There are some groups that get hung up on the 144,000 and completely miss the multitude. The language is intended to be inclusive: nations/tribes/peoples/languages should include someone from everyone.
The identification of the multitude is another occasion for worship. The multitude (dressed in white with palm branches) is identified as those who have "come out of the great ordeal". I think it would be an error, and many make it, that they were "rescued" from the great ordeal. I believe these are they who were martyred in the great ordeal -- who bore witness to Christ with their life and their death. They have now entered the eternal shelter (15) and the concluding poem echos the last chapter of the book:
"They will hunger no more, and thirst no more
the sun will not strike them, nor any scorching heat;
For the Lamb at the center of the throne will be their shepherd
and he will guide them to springs of the water of life,
and God will wipe away every tear from their eyes." Revelation 7:16-17