Do you wear glasses/contact lenses? If so, refraction is basically what strength your glasses are. The ATC medical requires between + and - 5 i think (im -3.5). If you dont wear glasses and can see 'perfectly' then this is likely to be 0 (though in truth its unlikely to be exactly 0, 20:20 vision is not 'perfect' vision, it is possible to be better, though few are!