A pronoun is a word that takes the place of a noun (or another pronoun) and refers, in a shorter way, to a person, place, thing, or abstract idea.
It's clearer to use pronouns instead of nouns in many cases:
Charlotte said that the book belonged to Charlotte.
Charlotte said that the book belonged to her.
There are many types of pronouns:
1) Subject pronouns can perform the action in sentences.
I/He/She/It likes ice cream.
We/You/They like ice cream.
2) Object pronouns have an action performed to or for them (and they make sense with the words "to" or "for").
Grandpa, who doesn't know anything about dietary fat or cholesterol and who doesn't want to know, gave the luscious, homemade peach and strawberry ice cream to me/him/her/it/us/them.
3) Possessive pronouns show that something belongs to someone.
Hey, get your @#@*! spoon out of that container; that ice cream is mine/his/hers/ours/yours/theirs.
No matter what type of pronoun you use, it needs to agree in: number (singular or plural), gender (masculine, feminine, neutral), and type (subject, object, possessive) with the noun or pronoun it replaces.