A heating element is part of an electric water heater that actually heats the water. In most residential water heaters there will be an upper and lower heating element. Heating elements in electric water heaters can often fail leaving you with very little hot water. If the upper heating element fails you could be left without any hot water at all. If the lower heating element fails you will usually have hot water but only for a brief period of time.
In order to replace the heating element in a water heater the power must be turned off and the unit needs to be at least partially drained. You can also test the heating element to see if it needs replaced before taking it out. Testing a heating element can only be done when the power to the appliance is off.