U - Unsprung weight
Unsprung weight is all of the weight that isn't carried by a car's suspension systems. It includes the wheels, brakes, tyres and some of the weight of the springs, shock absorbers (dampers) and suspension links.
The lower a car's unsprung weight, the less work the suspension has to do to keep the tyres in contact with the road. So, if a car's unsprung weight is low, it should handle better.