Follow Through and Overlapping Action are often seen as two separate principles though they are very closely related and both necessary to make animations seem more realistic
Follow Through refers to motion causing a reaction which continues even after the main action’s stopping point. An example of this is the kittens tails in the GIFs below as the tails continue to move after their bodies have stopped moving.
Overlapping Action is the idea that separate parts of a character or object will begin or stop moving at different times as the main action has an impact on the other parts of the character or object. For example, in this GIF from Natasha Allegri’s Bee and Puppycat, when she jumps her hair and arms also move as a reaction to this.