Reserve duty refers to a military service obligation where individuals, often part of the National Guard or Reserves, train periodically and can be called to active duty when needed. This system allows countries to maintain a ready force without the need for a large standing army. Members of reserve duty typically have civilian jobs or attend school while fulfilling their military responsibilities. They participate in training exercises and may be deployed during emergencies, natural disasters, or military conflicts, ensuring that they are prepared to support the regular armed forces when required.