Plain English Meaning

These are the roll-control surfaces near the wingtips that only work when the aircraft is flying slowly. At high speeds they are switched off, because using them out near the wingtip would flex the wing and could actually roll the aircraft the wrong way.

Full Definition

Ailerons mounted on the outer portion of each wing that are active only at low airspeeds, used to provide roll control during takeoff, approach, and landing on large swept-wing jet aircraft. At higher airspeeds these surfaces are locked out, and roll control is handed to ailerons and roll to prevent excessive () that the long, outer wing could otherwise produce.