Pronouns substitute a noun to avoid having to repeat it.

Without Pronouns With Pronouns
John loves bananas. John ate 10 bananas. John is now full. John loves bananas. He ate 10 of them. He is now full.

Personal pronouns

Singular Nominative Objective Possessive
1st person I me mine
2nd person you you yours
3rd person he she it him her it its
Plural Nominative Objective Possessive
1st person we us our ours
2nd person you you your yours
3rd person they them their theirs

Interrogative pronouns