Manner mode silences ringtones, notification sounds, and alarms, notifying only through vibration. Used on trains, in meetings, cinemas, and hospitals where sound would disturb others. "Manner mode" is Japanese-coined English; English speakers say "Silent Mode" or "Vibrate Mode."
Manner mode became ingrained in Japan due to strong social awareness of mobile phone etiquette in public spaces. Starting with JR East's 1999 campaign discouraging phone use on trains, "set to manner mode on trains" became cultural norm. Train announcements still request "please set phones to manner mode and refrain from calls."
iPhone uses the physical "silent switch" on the side. Android uses volume button long-press or settings menu. Advanced options include completely silent mode and "Do Not Disturb" that allows only specific contacts' calls.
Note that earthquake early warnings and J-Alert emergency broadcasts override manner mode with forced sound, ensuring life-critical information reaches users. See phone etiquette basics for more.