y=[zeros(1,2),ones(1,1),zeros(1,2)]
is just a way of writing
y = [[0, 0], [1], [0, 0]]
which then becomes
y = [0, 0, 1, 0, 0]
so it is just saying "two zeros, one one, two zeros".
Another way of writing would have been
t=-2:1:2;
y = zeros(size(t));
y(t == 0) = 1;
stem(t, y)
