Refrigerant is what makes air conditioning possible. It’s a chemical compound (the exact type and amount depends on your air conditioner) which shifts from liquid to gas at a comparatively low temperature and which pulls heat out of the surrounding air as it does so.  Centralized air conditioners work by cycling their refrigerant through a series of valves and coils. The refrigerant is first shifted into a liquid state and placed under a great deal of pressure (bleeding heat into the air outside your home in the process), then enters an evaporator coil where it shifts back to a gas (pulling heat from the air outside the coil). With the air around the system cools, a fan then pushes it into your home.