Follow through & Overlapping action
To fully understand, follow through and overlapping action, we need to look at real life. Different objects and materials have different densities and react in different ways to the environment around them. Take a rubber ball and water balloon as an example, a rubber ball would fall hitting the ground and roughly stay the same shape but a water balloon would fall and spread out in all directions along the point of impact. This would be an example of follow through, most objects and materials have different follow throughs. The key would be to understand the speed and movement of the object and ultimately hone your power of observation, watch real life and see how things act.
Overlapping action is an addition to this concept, take a man with a trench coat walking, the man would move without much movement to the body. However, the coat at the bottom would drag behind slightly.
https://garagefarm.net/blog/follow-through-and-overlapping-action-in-animation