A myriad is a lot of something. If you’re talking about Ancient Greece, a myriad is ten thousand, but today you can use the word in myriad other ways.
Myriad comes from the Greek myrioi, the word for ten thousand, or less specifically, a countless amount. Myriad can be a noun, like a myriad of choices, or an adjective, like when you study myriad subjects in college. If you lift a rock you might find a myriad of bugs. Sticklers often look down their noses at using myriad as a noun, but that usage came first.