The diesel glow plug controller, in conjunction with a relay, is the component responsible for powering the glow plug(s) when starting the engine. A faulty controller is usually identified if you are having a hard time starting your diesel engine. This will be most evident in the cooler months of the year. An additional symptom can also include large amounts of black smoke from the exhaust pipe during start up. Of course, if the controller has become completely inoperative, then the glow plugs will not receive any power, and the engine will most likely not start at all.
For more info, see How to Replace the Diesel Glow Plugs