Plain English Meaning

On a big jet, the roll-control surfaces nearer the body of the wing take over for the ones near the wingtip when the airplane is flying fast. They give the pilot a smaller, safer amount of roll movement at high speed.

Full Definition

A pair of ailerons mounted on the inboard section of each wing, closer to the fuselage, used for roll control during high-speed flight. On large jet transports, these are the only ailerons active at high speeds, while the outboard ailerons are locked out to prevent excessive wing twist and loss of roll authority.