A pronoun is a word that replaces a noun in a sentence. Pronouns are used to avoid repeating the same nouns over and over again. For example, "Jeremy ran so fast, you'd think his life was on the line." The pronoun "his" saved us from repeating the name Jeremy again.
Common pronouns include I, me, mine, she, he, it, we, and us. In truth, there are many different types of pronouns, each serving a different purpose.