A curfew was a order by a government or other authority given to the public to leave public areas during a designated time. Persons found to be in violation of the order are typically detained and arrested or forcibly taken to their homes. Curfews are typically instituted for the purpose of preventing criminal activity during times when there are few witnesses in public.
In 2370, Li Nalas, temporarily serving as first officer of Deep Space 9, recommended to Commander Benjamin Sisko that a curfew be instituted in order to prevent vandalism. He did so after several areas of the station, including a cargo bay, were vandalized with graffiti. (DS9: "The Circle")