Lembas, also called waybread in the Common Speech, is a special food made by the Elves. The cakes are very nutritious, stay fresh for months when wrapped in leaves, and are used for sustenance on long journeys. Lembas is a brownish colour on the outside and a cream colour on the inside. The secret of lembas is closely guarded, and only on rare occasions is it given to non-Elves. Like other products of the Elves, it is offensive to evil creatures; Gollum refuses outright to eat of it.
Galadriel gives a large store of it to the Fellowship of the Ring upon its departure from Lothlórien. One of the elves comments "[...] we call it lembas or waybread, and it is more strengthening than any food by men, and it is more pleasant than Cram, by all accounts." Frodo Baggins and Samwise Gamgee subsist on it through the majority of their journey from there into Mordor.
Tolkien most likely based lembas on bread known as hard tack that was used during long sea voyages and military campaigns as a primary foodstuff. This very un-magical bread was little more than flour and water which had been baked hard and would keep for months as long as it was kept dry. However, Heinrich Cornelius Agrippa wrote in his book Libri tres de occulta philosophia (Book 3, Chapter 13) of a herb from Scythia that allowed people to go for twelve days afterward without any need for food or water. It is also possible that Tolkien based lembas on this description in Agrippa's writings.
In Peter Jackson's Lord of the Rings movie trilogy, the term "lembas bread" is occasionally used. This is incorrect usage; but because the gift of lembas at Lothlórien is not included in the theatrical release of The Fellowship of the Ring, the redundant term "lembas bread" was probably chosen in order to immediately identify the substance to filmgoers at the beginning of The Two Towers.