The words are taken from early in The Fellowship of the Ring book - from the chapter "Three is Company" - written originally by Bilbo: the passage goes:

Home is behind, the world ahead, And there are many paths to tread Through shadows to the edge of night, Until the stars are all alight. Then world behind and home ahead, We'll wander back to home and bed. Mist and twilight, cloud and shade, Away shall fade ! Away shall fade ! Fire and lamp, and meat and bread, And then to bed ! And then to bed !

If you compare the song Pippin sings for Denethor in the film, you can see how Peter Jackson morphed Bilbo's original song into a rather doomful version. I like it, and the accompanying scene of the doomed Gondorian cavalry led by Faramir is one of the best in the Trilogy of films - VERY artsy.