Words called personal pronouns are used in place of nouns. It is based on various factors.
Person: first person (I, we), second person (you), and third person (he, she, it, they)
Number: singular (I, you, he, she, it) or plural (we, you, they)
Gender: masculine (he, him), feminine (she, her), or neutered (it)
Case: subjective (I, you, he, she, it, we, they), objective (me, you, him, her, it, us, them), and possessive (my, your, his, her, its, our, their)