Revelation 7 describes two armies—the 144,000 and the multitude from the nations. These two groups are identified in various ways.
Some have argued that they represent Jewish and Gentile believers respectively (dispenational).Others see them as two visions of the same group (non-dispensational).
Probably the best way to understand the vision, however, is as a recapitution of the earlier vision of the Lion and Lamb in Revelation 5:5–6. A number of reasons lead to this conclusion.*
First, the two visions alternate between John “hearing” and then “seeing.” In Revelation 5:5, John hears about the Lion but looks and sees that the Lion is actually the Lamb (Rev 5:6). In Revelation 7, John hears the number of the sealed (Rev 7:4) but looks and actually sees a multitude (Rev 7:9).
Second, both passages note the tribe of Judah. John hears “the Lion of the tribe of Judah” in Revelation 5:5 as well as hearing 12,000 “of the tribe of Judah” in Revelation 7:5.
Third, Bauckham notes, “To the Lamb standing (5:6), who has ransomed people from every tribe, tongue, people and nation (5:9), corresponds the multitude from all nations, tribes, peoples and tongues, standing before the Lamb (7:9).”
These correspondences at least suggest that Revelation 7 advances the messianic army motif that Revelation 5 had begun. If so, then Revelation 7 parallels the Lion and Lamb vision of Revelation 5:5–6. In that earlier vision, John hears about the Lion of Judah but actually sees the Lamb slain. The point is that the conquering messiah conquers through his sacrificial death.
The same relationship probably carries over to Revelation 7. John hears a conquering army of the Lion Judah as many Jewish believers hoped for a messianic conquest, but he instead sees the Lamb’s army whom the lamb has ransomed from the nations. Instead of arraying for battle, they array for worship.
Certain modern cults play up the idea of 144,000 sealed saints and integrate them into their religion. John tells a different story. The 144,000 are what John hears, but what he actually sees are saints from every, tribe, tongue and nation worshipping the lamb who was slain.
I am following the arguments made by Richard Bauckham in The Climax of Prophecy.