Differential is when the ailerons move a different amount up than down. The down moving aileron tends to cause more drag than the up moving aileron which ends up counteracting the turn command to some degree. This is mainly an issue in high-wing planes, like cubs. By programming the ailerons such that they go down only about 2/3s as much as they go up, this issue is reduced or eliminated and the plane responds to the turn command more smoothly.