Expertise in Timing comes best with experience and personal experimentation (trial and error!).
The basics are: more drawings between poses slow and smooth the action. Fewer drawings make the action faster and crisper. A variety of slow and fast timing within a scene adds texture and interest to the movement. Most animation is done on twos (one drawing photographed on two frames of film) or on ones (one drawing photographed on each frame of film). Twos are used most of the time, and ones are used during camera moves such as trucks, pans and occasionally for subtle and quick dialogue animation. Also, there is timing in the acting of a character to establish mood, emotion, and reaction to another character or to a situation. Studying movement of actors and performers on stage and in films is useful when animating human or animal characters. This frame by frame examination of film footage will aid you in understanding timing for animation.
Two objects of identical size and shape can appear to have vastly different weights simply by manipulating the spacing of their inbetweens. A heavy goods train with massive inertia, might take several kilometers of railway track to build up to its final running speed. This acceleration is long and slow. A mash mellow on the other hand, with practically no weight at all, might be shot from a gun and attain full speed within a few micro seconds. We can artificially represent these two types of movement through the way we use slow-in and slow-out.
The default setting for most time-based software packages designed to manipulate visual elements is to create strict mathematically even divisions between key positions. The result is very unnatural motion in that all objects instantaneously achieve full speed, or stop instantaneously. This works against almost everything we observe in nature and we read this type of motion as a ‘bump’ when its starts and a ‘bump’ when it finishes. This also applies to digital camera moves which can look particularly unnatural when no fairings have been used to start and stop the movement.