I assume you mean this effect: https://en.m.wikipedia.org/wiki/22%C2%B0_halo

It's due to the shape of the ice crystals. They reflect light to you at about a 22 degree angle relative to your point of view. So it's not the light reflecting back, rather it's being bent on its way to your eyes. At angles below 22 degrees, the light doesn't refract so the sky appears darker inside the ring.