Seeds have quite a bit packed inside the seed coat, including the endosperm, cotyledon, plumule and radicle; one will also find a hypocotyl or stem in a dicot seed. The terms monocot and dicot refer to the number of leaves a plant has when it first germinates. A monocot has one leaf, whereas a dicot has two, thus the need for the stem inside the seed.
Endosperm is found in all monocot seeds. It is the food source that temporarily keeps the plant embryo alive inside the seed. Without this food course, many seeds would not survive to be planted or germinate. The cotyledon in a dicot serves the same purpose as the endosperm in a monocot seed.