A cooling system in HVAC is a system that is designed to reduce the temperature of a space or area. It works by circulating air through a series of ducts and vents, which are connected to a cooling unit. The cooling unit is typically powered by electricity and uses a refrigerant to absorb heat from the air. The refrigerant is then compressed and circulated through a series of coils, which release the heat outside the building. The cooled air is then circulated back into the space, providing a comfortable environment.