Many verbs need an object for the sentence to make sense. The object is usually in the accusative case. You already know many verbs that take an accusative object: 

haben, lernen, brauchen, anrufen, essen, trinken, nehmen, suchen, kennen, machen, lieben, hassen, besuchen, besichtigen, bestellen, bekommen, mögen, putzen, kontrollieren, vorbereiten, waschen, öffnen, schließen, reparieren, küssen, werfen …

The direct object can be a noun with an article or a personal pronoun:  

Example:
Der Mann bestellt einen Kaffee. Er trinkt ihn und wirft den leeren Kaffeebecher dann in den Papierkorb.

Personal pronouns in the accusative

Every noun can be substituted with a pronoun. The personal pronoun can represent a person, thing or fact. Every pronoun can take the nominative or any of the other cases. For example, a personal pronoun will be in the accusative when it follows a preposition or a verb that takes the accusative.

Most personal pronouns in the accusative have a different form from the nominative, but some stay the same.