Revelation 15:3,4

3  And they sing the song of Moses the servant of God, and the song of the Lamb, saying, Great and marvellous are thy works, Lord God Almighty; just and true are thy ways, thou King of saints.

4  Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.

These that came out of the great tribulation sing the song of Moses and the song of the Lamb. The King of the Saints is Jesus. The words of the song that they sing are partly given in the verse. The song is one of victory like the song Moses wrote telling about Israel’s deliverance from Pharaoh’s army at the Red Sea (Pharaoh was a type or foreshadow of the Beast). God supernaturally delivered Israel and then destroyed Pharaoh and his army in the red sea. This was a foreshadow of the destruction of the Beast and the casting of the Beast alive into the Lake of Fire (a Red Sea).

They also sing a song of eternal redemption through the blood of the Lamb. In their song they proclaim the holiness of God and sing that all nations shall come and worship before Jesus after the final judgments are poured out.