"They have books."
Translation:Onların kitapları var.
It's the possessive suffix. Look at the Tips & Notes for the Possessive section.
For the 3rd person both singular and plural you just add a vowel at the end of the word according to 4-way vowel harmony (unless the word itself ends in a vowel, then you add a buffer -s-).
Since the last vowel in kitaplar is a, the the ending is ı.