Plain English Meaning
Briefly relax the back-pressure you're holding on the controls so the wing isn't being asked to lift as hard. The wing then starts flying normally again instead of stalling.
Full Definition
A control input — typically a brief reduction in back-pressure on the elevator — that decreases the wing's angle of attack and reduces the aerodynamic load (G-load) the wing is producing. Used to recover from or prevent an accelerated stall by allowing the wing to fly again before adjusting bank or power.
Context Anchor
You will hear this in stall recovery, steep turns, , and other situations where the is being pulled too hard or the wing is close to stalling.