They replace the noun. We can't speak easily without pronouns. It would sound like this: "Mandla took Mandla's keys and went to Mandla's car and Mandla unlocked the door of Mandla's car to look inside if Mandla's wallet was there." Pronouns are used to replace the noun. There are several pronoun types, like subject pronouns, object pronouns, possessive pronouns, relative pronouns, etc. You have to master these fluently because they are used just about every time you speak or write.