Simple answer - stand outside an air conditioner and feel the hot air coming off it? A heat pump just runs in both directions - hot in the winter and cold in the summer.
Longer answer - a heat pump literally 'pumps' heat. Even in freezing temperatures, 90% of the heat relative to room temperature is still in the air. In the winter time, the heat pump is cooling the outside air by absorbing some of that heat - and then transferring it inside. Which is the exact opposite of what happens in the summer.
Even more technical answer - refrigeration cycles work on the concept of compressing gas which makes it hot from all the work done (that's what you see on your electric bill) and it gets cooler (absorbs heat) when it is allowed to expand. When you blow on your hand through pursed lips, your breath feels cooler for this very reason.
A higher efficiency is obtained if the gas is compressed all the way into a liquid at ambient temperature - cooling the liquid creates the warm air you feel coming off the heat pump condenser coils. The liquid then goes through an orifice to expand it into a gas where it absorbs heat (which makes it feel cold). This is the cool air you feel off your air conditioner side in the summer.
There's even more to it than this - but I'll let one of the refrigeration experts handle that.
Keith & Liz
2010 SilverBack 33L