Plain English Meaning
The flap on one wing moves but the flap on the other wing does not, so one wing produces more lift than the other and the airplane wants to roll and turn toward the side with the smaller flap setting.
Full Definition
A condition in which the wing flaps on one side of the airplane extend or retract while the flaps on the other side fail to move with them, resulting in unequal lift between the two wings and a strong rolling and yawing tendency toward the wing with less flap deflection.
Context Anchor
Encountered during flap extension or retraction, especially when the airplane starts rolling unexpectedly after the pilot selects a flap setting.