And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth. (Revelation 14:3)
The experience is of such a personal nature that only those who pass through it can appreciate its significance. To them the song is a treasured and comprehensive summation of the experiences through which they have passed in the closing stages of the conflict between good and evil.