The meaning of the statement
average = sum ( sum ( x(:,:) ) ) / n / n;
is that whoever wrote the code knows very little about MATLAB. Assuming x is an n x n matrix, the code calculates the average/mean by summing over all the columns to create a row of sums and then summing across the row to get the total sum of the matrix. It then divides by n twice to get the average. The same thing can be done much more clearly with
average = mean(x(:));
where (:) is needed to turn a matrix into a column vector.
