There are a few different types of pronouns but possessive pronouns (mine, yours, his, hers, ours, theirs) express ownership of something. Pronouns take the place of nouns in a sentence. We use them a lot in spoken English and to avoid repetetion in written English.

For example: "My car is blue." I'm saying that the car belongs to me. I can replace "my car" with a possessive pronoun.

"What color is your car?"

"Mine is blue. What color is yours?" Since we are already talking about my car, it's understood that "mine" refers to my car. If I want to ask about the other person's car, I can use the 2nd person possessive pronoun. In this case, "yours" refers to your car.

Some other examples:

"My parents just had their house repainted. Theirs is the blue one at the end of the street."

"John's dog is named Peppy."

"Doesn't his wife have a dog too?"

"Yes, hers is named Spot. I like his better though." hers = her dog; John's dog = his dog = his