some line in firefly algorithm that i do not understand

hi, I would like to ask about below line in firefly algorithm

z=f(x,y);

what is the function of z in firefly algorithm?

why it must be the result of f(x,y)

while f is f=vectorize(inline(funstr))

f =

     Inline function:
     f(x,y) = exp(-(x-4).^2-(y-4).^2)+exp(-(x+4).^2-(y-4).^2)+2.*exp(-x.^2-(y+4).^2)+2.*exp(-x.^2-y.^2)

thank you.