A pronoun is a word that takes the place of a noun. Some common nouns are he, she, it, their, this, and you. The important thing is that pronouns must refer to a specific noun in the same passage. This specific noun is called the antecedent.
Choosing a pronoun can be difficult because there are several rules to pronoun - antecedent agreement. The pronoun will always refer back to the last noun of the same number (singular/plural) and gender. If the last noun with the same number and gender is not the antecedent, then the wrong pronoun has been used.
A pronoun must also refer to a noun antecedent in the same paragraph. You should avoid using a pronoun in the first sentence of any paragraph. It is also a good practice to use a pronoun no more than two or three times before restating the antecedent noun. Too many pronouns make for boring and repetitive writing.