This is a bit hard to explain. You must be wondering, what's the difference?
The key difference is this: the impact on your opponent may not come directly from you.
Direct impact on your opponent caused by you will come from the strength of your kicks and punches. But there is a limit to how much force you can generate, and how long you can sustain generating that type of force.
At the same time, your opponent also suffers damage when he hits something other that you. Like when he bangs against a wall, or falls on the ground. In some of these cases, the force is not generated by you and therefore not limited to your stamina/muscles. Such as when your opponent falls: the force is gravity. When your opponent overexerts himself, loses balance, and bangs against the wall: the force is from his own exertion.
So the key is not just what you can do to your opponent, but how you can use the entire environment, including your opponent, against him.