Ich warte auf den Bus.
English: I am waiting for the bus.
In German, some verbs are always used together with specific prepositions. These are called 'verbs with prepositions'. The preposition is important and changes the meaning of the verb.
Use this structure when a German verb always needs a certain preposition to make sense. The preposition often changes the meaning, and the following noun or pronoun will be in a specific case (Akkusativ or Dativ).
Ich warte auf den Bus.
English: I am waiting for the bus.
Sie denkt an ihre Freunde.
English: She thinks of her friends.
Wir sprechen mit dem Lehrer.
English: We are talking with the teacher.
Er interessiert sich für Musik.
English: He is interested in music.