Timing in animation

Basically timing is measured by how much motion occurs within each second of frames.

The quicker/slower an object or organism moves, the more the viewer is able to acknowledge its size, weight and unique characteristics, making it either believable or fantastical (something like what arril said!)

You get the idea that the ball slows as it reaches the peak of each bounce, giving it a physical weight that speeds up as it 'falls'