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The parhelic circle is produced when sunlight or moonlight reflects from near vertical faces of ice crystals.

Plate, horizontal columns and Parry oriented crystals all make it.

The reflection from the outside of a crystal or one more times within it.

The parhelic circle appears simple yet more ray paths contribute to it than in any other halo. Some are very intricate.

The circle is white (or very nearly so) because the angle of the exit rays is almost always equal to that of the rays entering the crystal. There is therefore no net refraction and colour dispersion produced as the ray enters is cancelled on exit.

via Parhelic Circle Formation ~ A halo of many paths

you can turn to wiki for a simpler explanation here