A morpheme is a unit of meaning within a word. Some words only have one unit of meaning while others have several.
A word like CATS has two morphemes CAT/S. These two morphemes tell you what the noun is and that there is more than one.
The morpheme TRACT means "to pull." Knowing this morpheme can help someone know that a tractor is meant to pull.
Many of the morphemes taught within the curriculum are Latin and Greek roots.