What does 3 ATM on a watch mean?

Protected in everyday life, bathing, accidental splashes, short swimming or car washing - it is resistant against perspiration, water vapor, rain drops.

It is tested for water resistance up to 3 ATM, and must be able to survive 30 minutes under water at a depth of 1 m (3 feet) followed by 90 seconds under a pressure corresponding to 30 meters.