Non-linear control is that the helicopter's reaction to joystick input is exponential. Thus the first movement doesn't effect the helicopter much, while more movement give sufficient response to the aircraft.
This gives the pilot the ability to have better control over the aircraft.
When the joystick input is 50%, the response to the aircraft is just 15%! Thus within 50% of the joystick movement range gives the ability to move within 15% of the helicopter movement range. Above the 60% of the joystick input will result in a more dramatic response to the aircraft. At 100% input the response to the aircraft is 100% as well.