The following is explained by using an Lg G3 (LG-D855) with Android 5.0 (UI may differ for other models)
Android 5.0 introduces new ways of managing sounds and vibrations when a call or notification arrives.
The following ‘Do not disturb’ options are possible from Android 5.0 (menu available using the device’s volume keys):
OFF (default setting): Sounds & vibrations are allowed for all incoming calls and notifications.
PRIORITY: sounds & vibrations are allowed for selected features only (picture 2a)
ON: no sounds & vibrations are allowed (not even alarms!)
This means that the ‘Silent’ mode, where only alarms are allowed, is gone in Android 5.0.