This badly written code recolour a rectangle of 4 rows by 8 column above the centroid of the largest objects. The red and green channels of that rectangle are set to 0, the blue is set to 255.
I say badly written since none of the for loops are required (the second one is particularly absurd). The first loop could just be replaced with:
[armax, armaxloc] = max(ar);
%no need to calculate mx (which by the way is just mx = numel(ar))
centroid = stats(armaxloc).Centroid;
The second loop is just:
i_aft(y-4:y, x-4:x+4, [1 2]) = 0; %set red and green channel to 0
i_aft(y-4:y, x-4:x+4, 3) = 255; %set blue channel to 255
Note that the above, just like the original code will give an error if the centroid is less than 4 pixels away from the left, top or right edge of the image.
Possibly, the original author intended to recolour a square around the centroid, but that's not what the code is doing.
