This is the 10388th most frequent Turkish word.
birisinin
"Birisinin" means "someone's" or "somebody's" in English.
In this sentence, 'birisinin' is used as a possessive form to indicate 'someone's' as the owner of the book.
Birisinin kitabını geri vermeliyim.
I need to return someone's book.
Here, 'birisinin' indicates possession, referring to what 'someone' has said.
Birisinin söylediklerini unutmamalıyız.
We should not forget what someone said.
In this context, 'birisinin' is employed to imply the need for assistance from 'someone.'