Ich warte auf den Bus.
English: I am waiting for the bus.
In German, some verbs always go together with a specific preposition. This is called 'Verben mit Präpositionen'. The preposition often changes the meaning of the verb, and you must learn which preposition goes with each verb.
You use this grammar when you want to say what you are waiting for, talking about, thinking about, or interested in. The verb and preposition always go together, and the object after the preposition is in a specific case (Akkusativ or Dativ).
Ich warte auf den Bus.
English: I am waiting for the bus.
Er denkt an seine Familie.
English: He thinks about his family.
Wir sprechen über das Wetter.
English: We talk about the weather.
Sie interessiert sich für Kunst.
English: She is interested in art.
Du fragst nach dem Weg.
English: You ask about the way.