Plain English Meaning
The flaps on one wing move but the flaps on the other wing don't, so the airplane wants to roll and turn toward the side with less flap because that wing is producing different lift and drag than the other.
Full Definition
A condition in which the wing flaps fail to extend or retract evenly on both wings, leaving more flap deployed on one wing than the other. The unequal lift and drag between the two wings produces a strong rolling and yawing tendency toward the wing with less flap, which the pilot must counter with aileron and rudder input.
Context Anchor
Encountered when using flaps before landing, including a or landing where steady and a predictable are important.