I sent my friendthe package. I sent the packagefor my friend. I sent the packageto my friend.
As you can see in the English examples above, an indirect object can either stand by itself (my friend), or be preceded by to or for.
In Italian, an indirect object must always be preceded by a preposition. If a pronoun is used as a substitute, you have two choices: a pronome personale indiretto: forma tonica or a pronome personale indiretto: forma atona.