Silent mode turns off a smartphone's ringtone, notification sounds, and vibration so the device makes no sound at all. Where manner mode announces a call by vibrating, silent mode suppresses the vibration too. It is meant for cinemas, theaters, exam halls, and other places where even the buzz of a vibrating phone is intrusive.
The name differs by platform. On iPhone, the setting that stops ringtones and notification sounds is called silent mode, toggled with the side switch or Action button or from Control Center; to stop vibration as well, turn off haptics under Settings, Sounds and Haptics. To keep notifications from arriving in the first place, use Do Not Disturb within Focus, which lets you allow specific contacts or apps through. Android calls it Do Not Disturb or Bedtime mode and can switch automatically on a schedule.
The important exception is Area Mail emergency alerts. Earthquake early warnings and tsunami warnings sound even when the handset is silenced. In airplane mode, however, radio transmission and reception stop altogether, so emergency alerts do not arrive either.
Using silent mode overnight has one weakness: you may not notice an urgent call. The iPhone's Do Not Disturb settings include Allow Repeated Calls, which is described as letting a second call from the same person within a short window (three minutes) past the notification suppression that Do Not Disturb applies. It only overrides Focus, though. If the handset itself is in silent mode with haptics turned off, that second call still produces no sound and no vibration. On nights when you do not want to miss an urgent call from family, leave silent mode off and instead set up the callers and repeated calls you allow through Do Not Disturb.
Silent mode is not, however, a defense against nuisance calls. It silences every caller without distinction, so calls you actually want disappear along with the rest. To stop particular numbers, use call rejection; to keep only unsaved numbers from ringing, a feature such as the iPhone's Silence Unknown Callers is more reliable. Silenced calls still appear in the missed-call log, so you can look up the number afterwards and decide whether to return it. For how to handle calls in different situations, see phone etiquette basics.