Possessive adjectives show ownership. In Spanish, they agree with the noun they describe — not with the owner.

Short Form (Before Noun)

• mi / mis — my
• tu / tus — your (informal)
• su / sus — his, her, its, your (formal), their
• nuestro/a / nuestros/as — our
• vuestro/a / vuestros/as — your all (Spain)

Examples

• mi libro — my book / mis libros — my books
• nuestro carro — our car / nuestra casa — our house
• su familia — his/her/their family

Clarifying Su/Sus

Since su can mean many things, add a clarifying phrase after the noun:

• el libro de él — his book
• el carro de ellas — their car

Reference:

Wikipedia: Spanish Grammar

https://en.wikipedia.org/wiki/Spanish_grammar