Lembas is a special type of waybread made by the elves. One small bite can fill the stomach of a grown man.
From The Tolkien Gateway Wiki:
Lembas was made first by Yavanna from special corn that grew in Aman, and Oromë gave it to the Elves of the Great Journey. For this reason, it was an Elven custom that only women should make lembas; they were called Yavannildi who knew the secret of its recipe. Also, the custom requested that only an Elven Queen should keep and distribute the lembas, for this reason she was called also massánië or besain.