Possessive pronouns in German are words that indicate possession. They answer the question "whose?" and either replace a noun or accompany it.
For example: mein Buch — my book, dein Auto — your car.
These pronouns are used to show who owns an object. Their forms depend on the person (I, you, he, etc.) and also change depending on the gender, number, and case of the noun.
It’s important to remember that a possessive pronoun must agree not with the owner, but with the object itself.
That is, if we are talking about "his book", the word "book" is feminine, so we say seine Buch.
This is an important part of grammar, without which it is impossible to build precise and clear sentences.