An electromagnetic field is an area or property of space generated by a moving electrical charge. A stationary charge will only produce an electric field, but if the charge is moving, then a magnetic field will also be produced. A high electromagnetic field could be an indication that something (or someone) comprised of energy is moving about.
EMF detectors were first introduced by KII Enterprises, not as a means of hunting ghosts, but rather as a way to measure high electromagnetic field levels which can cause negative health effects. These days, pretty much everyone on the hunt for ghosts has 1 or more in their toolbox.