What does impact mean? Is bullet impact the energy a bullet has when it hits the target? It has to be more than that but what is it?

Impact can be understood as three things combined.
1) The amount of energy imediatly transfered into the target. (imediatly destroying the tissue and delivering stretch wounds)
2) The amount of momentum transfered into the target (Delivering the jolt that can knock it over)
3) The damage cuased by the shot placement and the bullets performance. (If the shot is placed in the spine the Impact will be much more severe than if the shots hits the animal in the rump.

Hope that helps