reading assignment vs lesson

reading assignment and lesson both are nouns.

Word	Noun	Adjective	Verb	Adverb
reading assignment	Yes	No	No	No
lesson	Yes	No	No	No

As nouns, lesson is a hypernym of reading assignment; that is, lesson is a word with a broader meaning than reading assignment:

reading assignment (noun)	lesson (noun)
the reading of a passage assigned by the teacher	a task assigned for individual study
	a unit of instruction
	punishment intended as a warning to others
	the significance of a story or event