Passenger planes are powered by large jet engines under the wings. But these engines cannot start independently and are therefore assisted by a small engine at the back of the device.
The large jet engines under the wings of passenger aircraft can only start using a third engine in the rear of the aircraft.
Regular passenger aircraft have one large jet engine under each wing.
These engines suck air in at the front, which comes out again compressed at the back.
But for taking off a third engine is needed: the APU in the tail of the device.