please answer my question from code

hi,friends
in fingerprint codes,like this:

for i=1:W:w
for j=1:W:h
mean_thres = 0;

%white is ridge -> large

if i+W-1 <= w & j+W-1 <= h
mean_thres = mean2(a(i:i+W-1,j:j+W-1));
%threshold value is choosed
mean_thres = 0.8*mean_thres;
...
why we should start from a(i:i+W-1,j:j+W-1)?
and w=32?
thank u