The refrigerant begins as a liquid, and is pumped, by means of a motorized compressor, through tubes inside the fridge. This compresses the refrigerant and turns it into a vapor, pulling the warm air out of the fridge, leaving cold air in its wake. The vapor then passes through the thinner pipes outside the fridge, warming up, and turning to a liquid again. Warm air is released into the room, so you’ll often feel heat coming from behind or beneath the fridge. The whole cycle is then continuously repeated.
When the fridge reaches the desired cold temperature, the compressor shuts off. When the fridge warms up again, because it’s not completely air tight, or because you’ve been standing with the door open staring at the stuff in the fridge, deciding what to eat, you’ll hear the compressor come back on. We’re all familiar with the sound of the fridge “kicking in”.