We can measure variation in lots of different ways. Humans can be tall or short, skinny or fat, dark or light skinned, etc. It is best to think of variation as a distribution with an average and extremes.
Individuals vary on either side of the average.
Change operates on this variation. Change can either stabilize around an average or can favor an extreme. Stabilizing selection , such as that seen in the baseball example, works on the extremes. Directional selection favors one or the other of the extremes. Individuals with a particular trait will reproduce more offspring.