Lettered is an old, rather bookish way to say that someone is well educated — they have read a lot and they know learning and books. You meet it in older writing: “a lettered man” means a learned person, not someone who writes a lot of posted letters. Behind it sits an older sense of letters meaning education, the way we still say “a person of letters” for a writer or scholar. Today the word feels literary and a little stiff; in everyday talk we would say well educated, well read, or learned. The same spelling is also used for something marked with alphabet letters, like a lettered jacket — that is a different use, and not this one.