If you meant x=a(2:end) then that would contain 1, 2, 12, and 4.
When used in a single index, "end" stands in for numel() of the array it is being applied to. Your a is length 5, so a(2:end) stands in for a(2:numel(a)) which is a(2:5) . So the second, third, fourth, and fifth elements of a would be selected.
If you were using two dimensional indexing, like
b = [3 1 2 12 14; -4 9 3 8 2]
b(:,2:end)
then "end" stands in for size() of the index in that position. So b(:,2:end) would stand in for b(:, 2:size(b,2)) which would be b(:, 2:5) . The : in the first position would stand in for 1:end, as if you had written b(1:end, 2:end), so that would be b(1:size(b,1), 2:size(b,2)) which would be b(1:2, 2:5) and would give columns 2, 3, 4, 5 of both rows of b.
