Here we have Bob, and Kevin watching Christopher climb. Dad is working the fireman belay, while Kevin belays his brother.
A fireman belay is used when you are learning to belay. It serves as a backup, in case the belayer makes a mistake while mastering the technique. The fireman belay should always stand close and behind the belayer, giving them enough slack to allow the belayer to move the rope freely, while keeping the rope off the ground. If a belay mistake is made, the fireman belayer can pull the rope and activate the breaking mechanism on the belay device.
Once you have mastered the belay technique, a fireman belay is no longer required.