A PRONOUN is a word used instead of a noun. The noun in whose stead it stands is called its ANTECEDENT.
Ex: JOHN took Mary's BOOK and gave IT to HIS friend. In this sentence book is the antecedent of the pronoun it, and John is the antecedent of his.
PRONOUNS SHOULD AGREE WITH THEIR ANTECEDENTS IN PERSON, GENDER, AND NUMBER.