Any air conditioning unit consists of 2 coils, an evaporator coil and a condensing coil. The inner coil in the area in which you are trying to remove heat from is the evaporator. The evaporator coil is a low pressure refrigerant cooled copper coil that typically runs 40-50 degrees F refrigerant temperature while flowing through the coil (providing the incoming air temp is 65F+
The outside layer of the unit the condensing coil. In between the indoor (evaporator) coil and the outdoor coil is a compressor. Think about what an air compressor does. Same thing kinda, except when refrigerant is in a low pressure state (which it is while its inside) it is in a gas form. After it gets pulled to the compressor and spit back out, its a high pressure liquid form.
pressure increase = temperature increase
you now have an outer coil that is running 40~ ish degrees warmer than your indoor coil is (more if ur room is hot or it is hot outside, up to 120-130F is typical if its blazing hot inside or outside)
Hot goes to cold, Passing the air outside over it, so in the winter hitting a 80-120F coil, that cold air grabs that heat outa the refrigerant and it cools well.
If you dont have enough heat to satisfy the cold of the outdoor coil however, your indoor coil will go to <32F which will cause the water vapor around the evaporator coil to freeze, causing low airflow over the coil, causing more freezing.
a variable speed unit combats this by detecting a low head (outdoor high) pressure, and reducing the fan speed. This reduces the amount of cold air being circulated over the outdoor coil, reducing the amount of heat that bid bad mr winter is stealing from the refrigerant.