One of the gifts that Lady Galadriel makes to the Fellowship is the gift of Lembas or way bread.
The word "lembas" is derived from two Early Elvish words. The fist word, Lenn, means journey. The second word, Mbass, means bread. The two were put together and, in the mythology of Tolkien's world, the two words converged and blurred a bit to become the Modern Elvish word Lembas.
Lembas is described as being a thin cake that is wrapped in leaves. The leaves it was wrapped in tended to be Mallorn tree leaves. It was considered to be a very tasty cake and one cake could sustain a person for an entire day. Lembas bread helped to keep a person's spirits up and helped bring them healing as well.
Lambas, like other journey breads, would not go bad for a long time as long as the cake stayed whole and wrapped in its leaf.
There was danger in Lambas bread though, in that it could make a mortal wish that they were not mortal and could cause them to grow weary of mortal life.
Tolkien hints in his books that only good hearted people find the taste of Lembas to be pleasing. Both Gollum and the Orcs find the taste of the bread to be terrible despite the fact that all of the 'good' characters are heartened by the taste.
It is odd that Galadriel would give the Fellowship a significant supply of Lambas. Lambas was not normally given to mortals due to the afore mentioned dangers in it. One could guess that Galadriel, in her wisdom and mirror, saw the urgent need for the Lambas in the journey ahead for both the hobbits and the rest of the part.
Lambas is what makes it possible for Aragorn, Gimli and Legolas to chase after Merry and Pippin when they are taken captive. Without Lembas, there is no doubt that Frodo and Sam would have failed in their quest due to plain starvation.
Galadriel's gift of Lambas is frequently overlooked as being one of her notable gifts, but it very well may have been the most important one out of all of them.