Very commonly a sentence has an indirect object in addition to the subject and the direct object. The subject does the action, the direct object (whom or what) directly receives the action, and the indirect object describes to whom or for whom the action is done.

She  gives  the book  to Mario.
Sub.  V.      D.O.        I.O.

The subject performing the action is Ella, the action is the verb da, the direct object receiving the action is el libro, and person for whom the action is done is Mario. In all cases, the indirect object describes to whom or for whom the action is done.