Professional pilot slang for the flight simulator they must periodically (usually every six or nine months) visit, wherein they must thrash, sweat and strain with the aircraft equipment malfunctions, foul weather and Air Traffic Controller screw-ups that their sadistic Sim Instructor (chuckling "heh, heh")inputs at his panel in the rear of the simulated cockpit. The box can be a real E-ticket ride. Mounted on hydraulic struts/actuators that can produce pitch, roll and yaw, they are a treat to watch in action; a WHOLE LOT better than experiencing the reality from the inside!
Obviously, simulators save airlines money and are invaluable for practicing emergency procedures (see red box items and inculcating habit patterns that will serve a pilot in a real-world emergency.
Still, most pilots look forward to their time in "the box" with as much enthusiasm as they do going to the dentist for a root-canal!