C'est mon livre.
English: This is my book.
In French, 'adjectifs possessifs' are words used to show who owns or is related to something or someone. They are like 'my', 'your', 'his', 'her', etc. in English, but in French, they change depending on the gender and number of the noun.
You use French possessive adjectives to show that something belongs to someone or is related to someone. The form you use depends on the owner (person) and the gender (masculine/feminine) and number (singular/plural) of the thing owned.
C'est mon livre.
English: This is my book.
Voici ta maison.
English: Here is your house.
Il cherche ses clés.
English: He is looking for his keys.
Notre professeur est gentil.
English: Our teacher is kind.
Leurs amis sont ici.
English: Their friends are here.