A pronoun is a word used in place of a noun in order to avoid repeating the same nouns over and over again in a piece of writing. The noun being replaced is referred to as the antecedent of that pronoun. Each pronoun should refer back to a single antecedent.
One common example of a pronoun error is to not make the relationship between the pronoun and antecedent clear. This could occur when a writer is referring to more than one person of the same gender in a sentence or paragraph, and uses a pronoun like "he" or "she" which could refer to more than one person in the sentence. This creates a confusing pronoun ambiguity. In the sentence "Jim knows Bob, but he is a strange person," it is unclear whether Jim or Bob is the strange person because the pronoun "he" could refer to either Jim or Bob. It does not have a clear antecedent.