What does 5 ATM water resistant mean?

Water resistance is measured at a static or motionless state.

ATM or “atmospheres” is the amount of pressure a watch can withstand. 5ATM will withstand pressures equivalent to about 50 meters, or 164 ft under water. It is important to note that this is a measure of pressure NOT depth. 5ATM doesn’t mean the watch can be used diving to a depth of 50 meters.