Ease curves vary the timing of a function curve. An unaltered function curve charts the value of an animated parameter over time. An ease curve charts changes to the timing of the function curve. Changing ease curve values shifts the time of the original track left or right.
The horizontal scale of an ease curve represents normal time, just as it does for all function curves. The vertical scale of an ease curve represents the time scale of the function curve the ease curve is applied to. Changing the shape of an ease curve changes the way time is interpreted by the affected function curve.
The value of the ease curve at a particular frame is a frame value from the original track. For example, if the ease curve is 0 at frame 0 and 10 at frame 10, the original track plays at its original speed. If the ease curve value at frame 10 increases to 20, the original track plays to frame 20 by frame 10: it has been sped up by a factor of two.
Suppose you have animated a bird flying around the sky. After viewing the animation you decide that you want to change the position of the bird so it moves quickly at the beginning of the animation and then slows to a leisurely pace toward the end.
You could accomplish this change by editing position keys, ranges, and function curves but it would require more work than using an Ease curve. Applying an Ease curve to the Position track provides a quick and easy solution.
Dragging the Ease curve causes time to compress near the beginning of the animation and stretch out near the end. You can tell that the bird's motion starts out fast and then slows down by looking at the effect the Ease curve has on the Position function curves.