A pronoun is something like "he" or "she". It's a noun that refers to a person who is either the subject of the sentence, or the (in)direct object of the sentence.
A possessive pronoun is where you make the pronoun able to hold onto another object. For example, we change "she" to "her" to allow for a woman to gain possession of an object.
Example: She was about to go to the store, but she forgot her keys.
The word "her" is the possessive pronoun that has ownership over the keys.