A curfew was an order during which regulations applied. Generally a curfew was a period of time when a population was ordered to return to and remain in their residences. A curfew was often a component of any imposition of martial law.
During and following the crisis on Tarsus IV a curfew was ordered by the colonial government. In the wake of the murder of the 4,000 colonists by then Governor Kodos and his supporters, Starfleet and the colonial government kept the curfew in place in order to deal with the civil unrest that had arisen. (DSC novel: Drastic Measures)