В какой степени это может означать "научить нравственности людей". Это значит давать жизненные советы, которые люди воспринимают как выговор, назидание или проповедь. И обычно не всем это нравится.

When you think that someone behave in appropriate way you begin emphatically explain your own point of view as the only correct point of view. "How could you done that thing? It is unacceptable! You should have done so and so.. You should be ashamed of your own actions" Parents usually do so when they are disappointed with their children's behaviour. This is what "читать людям нравоучения" means. I am not really sure whether is the same thing as "teach moral to people". "teach moral to people" as for me has general notion which doesn't depend on situation. We can be tought moral at school or at home just as a part of our upbringing.

Учить других жить правильно.