What's the meaning of CADR rating?

clean an entire room, all of the air in the room needs to pass through the air cleaner's filter.
Air purifier are rated by the amount of air moving through the filter.
This rate is called CADR or Clean Air Delivery Rate.
CADR measures the amount of particle-free air being delivered into the room.

The higher the CADR rating, the larger the room size that an air cleaner can clean.