This is referring to the bit in RotK where Faramir tells Eowyn that he often dreams of the wave that drowned Númenor. This is that dream.
Tall wall, wave of glory
Rolling, foaming, wave of terror
Raising high, wave of glory,
Like a comb in mermaid hair.
Singing in unearthly tones,
Roaring now in rumble deep
Shaking down beneath my bones
No longer does the ocean sleep.
Green and blue, white foam above
Curved and shell-like, fragile not
No longer do Men the Valar love
The ocean will keep their bones to rot.
The wave comes nearer still
And from the walls men leap,
Yet no terror, only grief I feel-
As now I awake from sleep.