The sum(M) command computes the sum of the elements in the first non-singleton dimension of the given array.

The sum(M,dimension) command computes the sum of the elements in the given singleton dimension of the given array.

If M is 1-D, sum(M) computes the sum of the elements in the given vector.

If M is 2-D, sum(M) or sum(M, 1) creates a row vector of column sums of the matrix.  The sum(M, 2) command creates a column vector of row sums of the matrix.

When M has n dimensions, sum will return an array with n-1 dimensions.  Subarrays will be added together along the specified dimension (or first non-singleton dimension if unspecified).