A pronoun is a word that stands in place of a noun.
Peter is eight. He likes tennis.
In this example, the word he (a pronoun) is used in the second sentence instead of Peter (a noun).
Using the pronoun he sounds better than repeating the word Peter.
Peter is eight. Peter likes tennis.
A pronoun can also be used instead of a noun phrase (a noun made up of several words).
The dog with the fluffy tail loves chasing things. I have just thrown him a stick.
In this example the pronoun him is used instead of the noun phrase the dog with the fluffy tail.