Plain English Meaning
The boundaries — in speed, weight, and how hard you can pull on the controls — that the aircraft is built and certified to handle. Stay inside them and the aircraft performs as designed. Go outside them and you risk breaking it.
Full Definition
The maximum speeds, load factors, weights, and maneuvering boundaries within which an aircraft is approved to operate, as established by the manufacturer and published in the Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM). Operating outside these limits risks structural damage, loss of control, or aircraft failure.