Possessive determiners can be a substitute for definite or indefinite articles. They indicate that something belongs to someone or something else. Which possessive determiner to use in a sentence depends on the gender of the living being, thing or context to which the object belongs.

Examples:
Seine Stiefel waren hellbraun. (= The boots belong to a man. His boots were light brown.)
Ihre Stiefel waren hellbraun. (= The boots belong to a woman. Her boots were light brown.)

In German, the possessive determiners follow the same pattern of declension as the indefinite article "ein" and refer to the noun that they precede.

More:

German has four cases. In this chart you can see how the possessive determiners change according to case. In the singular the endings are the same as with indefinite articles.