Plain English Meaning
Ice is building up on the aircraft faster than the aircraft's ice protection equipment can deal with it, or it is forming in places that equipment cannot reach. The pilot must leave the icing area straight away.
Full Definition
An icing condition in which the rate of ice accumulation on an aircraft is faster than the airframe's de-icing or anti-icing systems can remove or prevent it, or in which ice accumulates in locations not protected by those systems. In this condition, immediate exit from the icing environment is required because the aircraft can no longer keep itself ice-free.
Context Anchor
Seen in instrument flying and weather discussions about ice buildup on airfoils during flight.