İ do not understand why they are used together (hayır and yok).

İ have a lesson that gives it like this "Bu evin üstünde adam var mı? Hayır, yok."

Also why is var used?

Thank you!  

Adam var mı? Hayır, yok. = Is there a man? No, there isn´t.

var= there is

Yok= there isn´t

Oh! I get it now! Thank you!

And "yok hayır" means a no altogether.

Geldi mi? = Has he come?

Yok, hayır = No