A cotyledon, or seed leaf, is a leaf that is stored in a seed. When the seed sprouts, the cotyledons are the first leaves that the plant has. Monocot plants have only one cotyledon, and other plants have two. Cotyledons often look very different from the other leaves, so they are not called true leaves.
Plants use cotyledons to make sugars through photosynthesis. They use the sugars to keep true leaves growing. After a while, the cotyledons often die because the plant does not need them anymore.