It is a way to measure the amount of air pressure in a hyperbaric chamber, Where normal breathing levels above sea start from 1.0 ATA around 14.7 pounds of pressure per square inch. If we were to dive into 33 feet of seawater, the sum of both the water and atmosphere’s weight would equal 2.0 ATA. 

In the chamber this is replicated through the cobination of an airtight container and continuous flow of pressurized air.