Most of the vehicle's weight/mass rests on the suspension system. The wheels, brakes, and some other components that are directly affected by the driving surface (particularly, changes like bumps and potholes) are considered unsprung mass, or unsprung weight. This mass moves and reacts differently than sprung mass, and resists changes in speed and direction. The material which spins with the wheel also affects rotational inertia.
There are some components that are only partially unsprung mass, such as suspension arms and struts/springs themselves. Unsprung mass reacts to the driving surface differently than sprung mass. A lot of weight is hard to move, so when unsprung weight encounters a bump in the driving surface, a heavy mass would adjust more slowly to the surface change, meaning reduced contact with the driving surface (less grip).
Reducing Unsprung Weight
Due to the benefits of having lighter unsprung components, many high-profile racing teams and tuning companies put a lot of research time into figuring out new ways to reduce component weight without losing the durability required for performance driving. This is especially the case for rally cars which are almost constantly driving over rough terrain which is an extreme test of the suspension's durability and responsiveness.