Pressure - Watches usually indicate their water resistance by a number on the case back or dial. The number or rating is normally a depth in meters, feet, ATM or Bar. Meters and feet are self explanatory. ATM (or Atmospheres) and Bar are synonymous within the industry.
The accepted rough industry conversion is; 1 meter = 3.3 feet =0.1 ATM or Bar. Example; watch rated at 30 meters = 100 feet = 3 ATM or Bar.
Pressure or depth ratings for watches are an odd thing to translate to the average person’s use. In other words why can’t I swim in a pool with a watch rated for 30 meters? The water resistance rating for the watch is the depth the watch’s water resistance could withstand in laboratory testing where the watch and the water are perfectly still. Movement of the watch while swimming or diving into water greatly increases the pressure on the watch, possibly beyond the depth rating for that watch. This would cause a leak.