The Do Not Disturb mode is a setting that mutes all sounds and vibrations and dims the screen, making it a very useful option when wearing the watch in, for example, a theater or any environment where you want the watch to operate as usual, but silently.
To turn on/off the Do Not Disturb mode:
If you have an alarm set, it sounds as normal and disables Do Not Disturb mode unless you snooze the alarm.