Plain English Meaning
Holding the control wheel turned into the wind during a crosswind takeoff, so the wing on the windy side stays down. As the airplane speeds up and the controls bite better, you ease off how far you're holding it.
Full Definition
The control input applied to the ailerons by deflecting the control wheel or stick toward one wing, used during a crosswind takeoff roll to keep the upwind wing from lifting. Full deflection is held into the wind at the start of the takeoff roll and gradually reduced as airspeed builds and the ailerons become more effective.