I'm assuming this means the average (or mean). In that usage, it is a case of the bra-ket notation introduced by Dirac.
For a matrix, just use mean() with an optional dimension input to tell whether you want to take the mean columnwise, or by rows.
A = randn(10,10);
mean(A) % gives a 1x10 vector of column means
mean(A,2) % gives a 10x1 vector row means
mean(A(:)) % or mean(mean(A)) gives you the mean of all elements
