Read the documentation of the max function. When applied to a 2D matrix, max returns a vector of the maximum value of each column. Hence, your description says to apply a second time to get the max of this vector:
max(max(your2Dmatrix))
A better way of doing this would be to actually reshape the matrix into a vector and take the max once. That works regardless the number of dimensions of the matrix, whereas the above only works with 1d and 2d matrices:
max(yourmatrix(:))
