Where -s(i) are using?

I saw hangisi, what it mean?

Thanks.

Hangisi means ´which of them´, ´whichever´.

http://tureng.com/en/turkish-english/hangisi

The -(s)i is in this case a possesive adjective and means ´his´ or ´its´.

http://www.turkishlanguage.co.uk/possadj.htm

Hangisi means which of them as JNQ wrote, -si is the equivelant of "of" in Turkish.

Thank you.

yes think of it as which (specific) one of them / which of theirs / them

(onların) hangisi

onlar-ın hangi-s-i 

Thank you.