In English, personal pronouns are used instead of nouns for 'people' and 'things'.
They are used so that we don't repeat the same words over and over again. Let's look at an example using the noun picture:
"Peter picked up the picture. He looked at the picture and then he put the picture down." It sounds unnatural to repeat picture too much, so we use the pronoun it instead:
"Peter picked up the picture. He looked at it and then he put it down." See? Much better! We use the picture at first so that people know what we are talking about. After that we know that it is the picture.