in searching for a good description I found this on the following website(not trying to plagiarize) :
https://rallyways.com/10869/lightweight-wheels/
1. Sprung Weight / Mass
The sprung weight is the total weight of all parts on the car that is supported by the car’s suspension. Some examples are the engine, exhaust system, bodywork and interior parts.
2. Unsprung Weight / Mass
The unsprung weight is the total weight of parts that are NOT supported by the car’s suspension. We’re talking about the shock absorbers, springs, suspension links, axles, the braking system and of course also wheels and tires.
Now, it is the unsprung weight of the vehicle that directly affects driving feel of a car. The smaller the amount of unsprung weight, the more effectively the suspension will be able to do its “stabilizing” job on uneven roads. So reducing the unsprung weight will result in an overall better handling car with a better driving feel.
3. Rotational Weight / Mass
Additionally, we have the rotational weightof the car. These are the parts that rotate, such as the drive shafts, wheels and tires. The amount of rotational weight will play a big role in the performance of the car. The lower the weight of these components, the less energy is required to accelerate and decelerate. This will enhance the car’s performance. Additionally, the further away from the rotational axis the weight is located, the more of a negative impact it will have on performance.
For example, a wheel that carries more of its weight around the outer edge of the wheel will perform worse than a wheel of the same weight that has most of that weight closer to the hub.
Bottom-line, when installing heavier wheels, you are actually adding unsprung and rotational weight to your car, which will have a bigger negative impact on the handling and performance of your car than any other similar weight added to the car elsewhere.