How filter2() is working in this code?

    function G=costfunction(im) 
            G=zeros(size(im,1),size(im,2));
            for ii=1:size(im,3) 
                G=G+(filter2([.5 1 .5; 1 -6 1; .5 1 .5],im(:,:,ii))).^2; 
            end
    end