TEMPO is a series of numbers written in the format #-#-#-# to direct how quickly you move through a single repetition of an exercise. It indicates how many seconds you take to move up and down and how many seconds you pause at the top and bottom of the movement. Each # represents the number of seconds.
The first # is how many seconds to perform the lengthening portion of the movement.
The second # is how many seconds to the pause at the end of the lengthening movement.
The third # is how many seconds to perform the shortening portion of the movement.
The fourth # is how many seconds to pause at the end of the shortening movement.
Let's use a bicep curl as an example. You need to figure out when a muscle is lengthening and when a muscle is shortening. In the bicep curl, the muscle lengthens when the forearm is traveling away from the upper arm. It is the movement that stretches the muscle. And in opposite fashion, the shortening portion of the movement is when the muscle contracts. So if the TEMPO is listed as 3-0-1-0, you would spend 3 seconds lowering the forearm and not pause at the 'bottom' of the movement, and take 1 second to bring the forearm back up, and not pause at the 'top' of the movement.
Here is a helpful video that also explains how to apply the TEMPO values to your exercises: