When people are didactic, they're teaching or instructing. This word is often used negatively for when someone is acting too much like a teacher.
When you're didactic, you're trying to teach something. Just about everything teachers do is didactic: the same is true of coaches and mentors. Didactic is often used in a negative way. If you heard that a movie is overly didactic, that's probably not good. Most people want to see a story and be entertained when going to the movies, and if it feels like the movie is just telling you what to think, that's didactic in a bad way.