Control reversal is an adverse affect on the controllability of some aircraft that occurs at high speeds. To the pilot it appears that the controls have reversed themselves, in order to roll to the left for instance, they have to push the control stick to the right, opposite of the normal direction.
The problem occurs when the amount of airflow over the wing becomes great enough that the force generated by the ailerons is enough to twist the wing itself. For instance when the aileron is defelected upwards in order to make that wing move down, the wing twists in the opposite direction. The net result is that the airflow is direct down instead of up, meaning that the wing moves upward, opposite of what was expected.
This problem was notorious on the Supermarine Spitfire. A series of upgrades to the wing structure helped eliviated the problem to some degree, but improvements in engine power typically offset it. This continued until a new internal design was introduced on the late-war Mark XIV version that cured it entirely.