Er interessiert sich für Musik.
English: He is interested in music.
A Präpositionalobjekt in German is an object that is connected to the verb by a preposition. Some verbs in German always need a certain preposition with their object.
Use a Präpositionalobjekt when a verb in German requires a specific preposition to complete its meaning. You cannot leave out the preposition, and you must use the correct case (Akkusativ or Dativ) after the preposition.
Er interessiert sich für Musik.
English: He is interested in music.
Wir warten auf den Lehrer.
English: We are waiting for the teacher.
Sie spricht mit ihrer Mutter.
English: She is talking with her mother.
Ich erinnere mich an den Film.
English: I remember the movie.