And they sang a new song before the throne and before the four living creatures and the elders. And no one could learn the song except the 144,000 who had been redeemed from the earth. — Revelation 14:3

The redeemed sing a new song before the throne — and only they can learn it. It's the song of those washed in the blood of the Lamb. Christ opened a new and living way through the veil, His flesh. We're saved not by works of righteousness but by His mercy and the washing of regeneration. By grace you've been saved through faith — it's the gift of God.