Someone or something moralistic has very strong beliefs about right and wrong, and uses them to judge how other people live. It is not the same as simply having morals — most people do. A moralistic person, speech, book, or film keeps pointing at other people’s behaviour and calling it bad. The word is almost always a criticism: it says they are preaching, or that they care more about laying down rules than about understanding anyone. You meet it in fairly educated English; in everyday talk people might just say someone is preachy or always judging.