Timing is the part of animation which gives meaning to movement. Movement can easily be achieved by drawing the same thing in two different positions and inserting a number of other drawings between the two. The result on the screen will be movement, but it will not be animation. In nature, things do not just move. Newton's first law of motion stated that things do not move unless a force acts upon them. So in animation the movement itself is of secondary importance; the vital factor is how the action expresses the underlying causes of the movement. With inanimate objects these causes may be natural forces, mainly gravity. With living characters the same external forces can cause movement, plus the contractions of muscles but, more importantly, there are the underlying will, mood, instincts and so on of the character who is moving.
In order to animate a character from A to B, the forces which are operating to produce the movement must be considered. Firstly, gravity tends to pull the character down towards the ground. Secondly, his body is built and jointed in a certain way and is acted on by a certain arrangement of muscles which tend to work against gravity. Thirdly, there is the psychological reason or motivation for his action—whether he is dodging a blow, welcoming a guest or threatening someone with a revolver.
A live actor faced with these problems moves his muscles and limbs and deals with gravity automatically from habit, and so can concentrate on acting. An animator has to worry about making his flat, weightless drawings move like solid, heavy objects, as well as making them act in a convincing way. In both these aspects of animation, timing is of primary importance.