Follow through and overlapping
The action of Follow Through and Overlapping refers to two closely related techniques which help to ensure movement has realism to it, and to make sure it seems as if the objects and characters abide by the laws of physics, including the principle of inertia.
Follow Through is the idea that loosely connected parts of a body or object will continue moving after the character has stopped.
Overlapping Action is a similar idea in that it describes how different parts of a body or object tend to move at different rates.
Arcs
Arc is the movement in circular paths.
Human beings do not move in one straight direction. They move in arcs. In the same way, the arc technique is to help animation create that illusion of movement.
Without arcs, the animation will look stiff and mechanical.
Slow in slow out
Slow in slow out is when an object or character in an animation starts its movement slow, gains speed, then decelerates. This is so that that the beginning and end of the movement is softened.