What is betterment?

Some items on a vehicle have a limited lifetime.  For example, the tires on your car do not last forever.  Should a tire for example be damaged in the accident requiring replacement and the tire has worn through half the tread, the insurance company will only pay for 50% of the cost to replace the tire since it is half worn out.  You would be responsible to pay for the other half of the tire - this is betterment...  You are improving the car with a new tire where a half worn out tire once was.