Decompression (in diving also colloquially called: “deco”) in physics designates the enlargement of the volume of a body by the reduction of the ambient pressure. In short: When the air pressure diminishes, the body expands.
Decompression while diving
When descending, the ambient pressure rises. Thereby, the breathing gases in the lungs are released into the body. During ascent, the pressure decreases again and the body expands. Thus the released gases must go back into the lungs. If you ascend too quickly, the gases cannot be eliminated properly, and instead little bubbles are formed. These bubbles are very dangerous and can cause deco sickness.
The formation of bubbles can be prevented by slowly ascending and doing stops time and again. In this way, the body can slowly transfer the gas back into the lungs. The right ascent speed is decisive thereby.