That's your pressure switch . When you remove the cover you'll be able to see a set of contact points . When the points are closed ( together ) the pump will be running . When the pump kicks off the points will be open . These points usually arc as they open & close & over a period of time will become burned or pitted . These points are what you need to check when you look in the switch . Also the small pipe the switch is mounted on sometimes clogs up & causes erratic operation of the pressure switch . If you end up taking the switch off use a screwdriver or something & run it down inside the small pipe & work it around a little bit to make sure the pipe is clear . Don't forget to turn off the power before checking the switch .