Spine refers to the deflection (stiffness) of the arrow shaft. The shaft has to be stiff enough for the draw weight of the bow and the length of the arrow, point weight and a couple other minor things. If the arrow isn't SPINED stiff enough it'll bend too much coming out of the bow. As it bends the blades on the front turn slightly sideways to the arrows intended travel which causes it to plane off track. The arrow eventually straightens out with the help of the fletching, but depending on how UNDERSPINED it is it may take too long to recover. The result is an arrow that hits the animal partly sideways thus impeding penetration.

Does this help a little?