Slow In and Slow Out
Movement usually starts and ends slower than movement in between. Objects accelerate and decelerate. They ease into and ease out of the action.
Your car isn’t doing 60 mph the instant you step on the gas pedal. It doesn’t go back to 0 mph instantly when you step on the break. It accelerates to 60 and decelerates back to 0.
In practice, this means adding more keyframes to the start and end of an animation. It’s not uncommon for anticipation to take up 25 percent of an animation and for follow through to take up a similar amount of time.