Hay is a type of dried grass that's fed to certain farm animals. You might buy bags of hay to feed your pet rabbit, who likes to nibble on it.
Hay is a mixture of grassy plants grown in fields, cut or mown, bundled in bales, and stored until ready to be fed to livestock. Hay and straw aren't the same thing; hay is cut when it's green, while straw is made up of the leftover dried stems and leaves of plants that have been harvested. And while some animals feed on straw too, hay is more nutritious. The Old English root is heg, "grass cut or mown."