Air source heat pumps absorb heat from the outside air. Very cold gas is pumped out, and absorbs a little of the heat from outside – even on cold days.
The gas is then compressed, which turns that small gain into much higher temperatures. The heat is then used to heat your radiators, hot water and underfloor heating system.
The heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its inside. In the same way a refrigerator works by taking heat from inside the box and dumping it outside your house. A heat pump take heat from the air and puts it in to your house.
It can get heat from the air even when the temperature is as low as -15° C!