Quiet hours are the local hours, typically late evening to early morning and sometimes Sundays, during which marketing SMS must not be delivered in a given country; transactional messages are exempt.
Many countries regulate or strongly expect that promotional messages arrive during waking hours. France, for example, holds marketing outside 08:00 to 20:00 and on Sundays. The rule is local time in the destination, which makes a multi-country campaign a scheduling problem rather than a single send time.
Codes, alerts and receipts are expected at any hour; a login code held until morning helps nobody.
At SMSMeteor: quiet hours apply by default per destination, editable per campaign; window.quiet_hours: bypass marks traffic transactional, and each country page states its default window.