This step is needed to make the left side of the equation a perfect square. This is done by halving the linear coefficient, squaring that value, and then adding it to both sides of the equation. In this case you would add:
(-16/2)^2=(-8)^2=64
So you would add 64 to both sides...
x^2-16x=-8
x^2-16x+64=56 now the left sides is a perfect square...
(x-8)^2=56