It's a device for re-establishing a normal heartbeat when someone has just suffered a heart attack. You see them around in most public places and workplaces. Once applied to the patient their operation is completely automatic and anyone can use one with minimal instruction. Thanks to the personal initiative of a few individuals, Hunsdon has a defibrillator that is always accessible, and the necessary training is available (free of charge) from a qualified first-aid instructor, David Robinette, contactable at firstname.lastname@example.org or 07977 779156.
Yes - someone should call an ambulance, but time is absolutely critical when someone has suffered a heart attack - look at the figure below.