A flight simulator is a full copy of an aircraft cockpit that moves and behaves like the real aircraft, where pilots train and are tested at regular intervals. The pilots sit in the real seats and use the real controls, but the aircraft never leaves the ground.
The simulator has three main parts. The first is the cockpit, an exact copy of the aircraft's, with the same screens, switches and levers. The second is a platform that tilts and shakes the cockpit, so the pilots feel take-off, turbulence or landing. The third is software that models the aircraft: how it responds to the controls, how its engines behave and what happens when a part fails. The software also draws the view out of the windows, such as an airport, clouds or a storm. Simpler training devices have the same cockpit and software, but no moving platform.
The main reason for a simulator is that pilots can practise things they should never try in real life. For instance, an instructor can fail an engine just after take-off, fill the runway with fog or switch off the instruments. The pilot must handle it, with no danger. A simulator is also far cheaper than flying: it uses no fuel and no aircraft is taken out of service. Airlines also use it for the regular tests that the regulator requires. The regulator checks and approves each simulator, to confirm that it behaves like the real aircraft closely enough to be used for the tests.
Here is a typical case. A pilot of a narrow-body airliner in Gurugram spends four hours in the simulator every few months. The instructor gives scheduled emergencies, and the examiner marks how the pilot responds. If the pilot fails one, the pilot trains again before flying passengers.
The simulator reuses the aircraft's avionics software or a copy of it. The same idea is used to train plant operators, as in the operator training simulator (OTS), and to train soldiers, in military simulation and training. Pilots' qualifications are one kind of professional certification. It belongs with aircraft maintenance and flight operations.