'It is a song to Elbereth', said Bilbo", and at the very end of the chapter there is a hint as to its meaning: "Good night! I'll take a walk, I think, and look at the stars of Elbereth in the garden. Sleep well!"
"It is a song to Elbereth," Bilbo said, and at the chapter's end, there was an indication of its meaning: "Good night! I plan to take a walk and gaze upon the stars of Elbereth in the garden. Sleep well!"