As the animation processes developed, systems needed to become implemented to help animators chart out the motion of an object. Timing charts are used to show the planned timing for a movement. But before we can understand this method you need to know some basic ideas of how the animation movement is planned out.
Key drawings, or also called extremes, or key poses in 3d animation, are the main drawings/ poses that illustrate the beginning and ending point of a motion. The most extreme poses of the object for a specific movement like walking, or lifting a glass, or a ball bouncing up and down, etc.
The inbetween drawings are the drawings placed in between the two extreme drawings.
The timing chart is used to determine the position of the inbetweens for the extreme drawings.
Basically, it indicates the small incremental movements that get you from one extreme to another. The greater the distance, or amount of change, from one frame to another will indicate how fast or slow the object seems to move.
Common timing charts are shown below:
Halves: Halves place an inbetween directly between the two extreme drawings/ poses.
Thirds: There are only two inbetweens placed a third of the way from the first extreme and a third from the last extreme.
Favors: A favor is an inbetween that is added closer to one of the extremes. This will add an easing in or easing out or sometimes called a "cushion" to the motion. The actual position is up to the individual animator. It has to "feel right" to be correct. This can be a very subtle thing and you will become more aware of its usefulness with more experience.
1/4's, 1/8's, 1/16's...: It is also common to see smaller divisions of the movement. The smaller the spacing on a chart the slower the motion will seem.
Timing charts will define the type of motion an object makes. It can be a linear mechanical movement. That is represented by equal divisions on a timing chart:
Example Linear/ Mechanical
or it can be an organic movement that utilizes easing in and out or a variety of spacing combinations.
Example Organic/ non linear motion:
For further discussion on the basics of animated motion watch the following films by award winning animator Norman McLaren.