A cotyledon is a seed leaf. A dicotyledon is a plant that has two leaves directly after it germinates. If you peel/open a dicotyledon seed, it will naturally fall into two halves which would become the cotyledons after germination. Good eaxmple that this is easy to see this in - peas and beans.
A monoctyledon has a single leaf directly after germination - examples - grasses and palms.