A heat pump in very simplistic terms is an air conditioner operating in reverse. An air conditioner cycle extracts heat or heat energy from the interior of a dwelling, to the exterior, so as to cool down the interior to a desired temperature.
The heat pump simply replaces the energy guzzling electrical element in your geyser that can use up to 40% of your household electrical power.
The reason why heat pumps are so efficient is because heat pumps do not create heat energy like an electrical geyser using an element. A heat pump transfers or pumps heat energy from one source such as the environment, to another source, such as a body of water in a geyser. This is the reason why heat pumps are referred to as a pump, a heat pump, pumps heat, it does not create heat.