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

I saw Birincisi and doğrusu. Why we add -(si)?

We add -si because we need these words to have possessive suffix. Biricisi actually means first of them, doğrusu means truth of them.

Thank you.