This number tells you how many cubic feet of air the air purifier runs through its filters in 1 minute of time.
Circulation numbers can vary depending on the fan speed. Many units have variable fan speeds with higher speeds circulating more air per minute.
Typically higher fan speeds cause more noise and decrease the life of the air filter due to more air passing through it.
However higher speeds often clean the air in a room faster and more effectively since they can draw air that is further away from the unit better than lower speeds.