learning vs letters

learning and letters both are nouns.

Word	Noun	Adjective	Verb	Adverb
learning	Yes	No	No	No
letters	Yes	No	No	No

As nouns, letters is a hyponym of learning; that is, letters is a word with a more specific, narrower meaning than learning:

learning (noun)	letters (noun)
the cognitive process of acquiring skill or knowledge	the literary culture
profound scholarly knowledge	scholarly attainment