Ce livre est le mien.
English: This book is mine.
In French, 'pronoms possessifs simples' are words that replace a noun and show to whom something belongs (like 'mine', 'yours', 'his', etc.). They agree in gender and number with the noun they replace.
Use French possessive pronouns when you want to say that something belongs to someone, but you don't want to repeat the noun. For example, instead of repeating 'la voiture de Marie', you can say 'la sienne'.
Ce livre est le mien.
English: This book is mine.
Voici ta chaise et voilà la mienne.
English: Here is your chair and there is mine.
Nos enfants jouent avec les leurs.
English: Our children are playing with theirs.
C'est ta clé ou la sienne ?
English: Is it your key or his/hers?