We already had a short glance at the personal pronouns in the chapter 4 - personal pronouns in basic form. And we have seen that personal pronouns can substitute the subject of a phrase.

Mary is beautiful. She is beautiful.

She substitutes the subject of the phrase Mary. So far, so good. Not only can the subject of a phrase be substituted by a personal pronoun, but also the objects of the phrase (direct object and indirect object). And you will see it's not so different from the English logic.

I give money to the men. = Ich gebe den Männern Geld. I give it to them. = Ich gebe es ihnen.