Having trouble understanding some code

   %
[col,row]=size(bw);
prj=zeros(col,1);
for i=1:col
    for j=1:row
        if bw(i,j)==0
            prj(i)=prj(i)+1;
        end
        plot(prj(i));
    end
end

The above code is to do a horizontal projection on an image, however, I am not sure how it works and I try to make a plot for the nested loop to get a better understanding but its crashing my computer. Any one kind enough to share some insight into how this works? Help much appreciated, thanks.

-AT