The abs(M) function computes the element-wise absolute value of M.  The result Matrix, R, is formed as R[i,j] = abs(M[i,j]).

$\mathrm{with}\left(\mathrm{MTM}\right)\:$

$M≔\mathrm{Matrix}\left(2\,3\,'\mathrm{fill}'=-3\right)\:$

$\mathrm{abs}\left(M\right)$

$\left[\begin{array}{ccc}3& 3& 3\\ 3& 3& 3\end{array}\right]$