The brig is the detention area aboard a starship, where prisoners and security risks are held behind forcefields or locked doors.
Starfleet ships keep holding cells for prisoners, hostile visitors and crew members confined for disciplinary reasons. In later eras the cells were sealed by forcefields rather than bars, letting guards see and speak with detainees.
The brig is overseen by the ship's security department. Prisoners have often escaped during emergencies, power failures or with outside help, making the brig a frequent setting for tension aboard ship. Worf on the Enterprise-D and Tuvok on Voyager both oversaw such facilities as heads of security.