Simply put, cotyledons are embryonic leaves of the plant.
Here's a better detailed diagram:
|Source on Britannica|
Notice again that the dicot has 2 cotyledons. Epicotyl will give rise to the shoot system and is above the cotyledons, while the hypocotyl is below the cotyledons and also contributes to the shoot. This is easy to remember because "epi" means above or upon, while "hypo" means below.
The radicle gives rise to the root system.
Here's a fun picture showing a peanut. The yummy part we eat is actually the cotyledons (leaves!). I never knew that little nub in the middle that I sometimes discard was the actual plant in the making. :)
Key Terms in my own words (clicking on the term will take you to the wikipedia page for more info) *PLEASE do not copy/ paste this to plagiarize homework assignments. Read about it, rephrase in YOUR own words. Don't let me be a source of academic dishonesty, even though I know a simplified list like this lends itself to that. Please, please... be a smart cookie.
Cotyledon: embryonic leaf
Dicot: a category of plants which has 2 cotyledons in its embryonic stage
Monocot: a category of plants which has 1 cotyledon in its embryonic stage
Radicle: embryonic root. First thing to emerge from seed.
Epicotyl: gives rise to shoot system, is above the cotyledons.
Hypocotyl: is below the cotyledons, contributes to the shoot system, adds height to the plant