Bouncing, also known as vertical displacement or oscillation, tells how much your (upper) body moves up and down while you are running. The measure is given as centimeters (cm).
The recommended range for bounching is dependent on your speed, step length and body height. The bouncing should not exceed of 4% of your height and 5% of your step length.
Excess vertical movement, i.e. up and down movement, wastes energy and reduce running efficiency.
John Foster and Inform Running introduce great 4 cues for better running gait in their web page