What a buckwheat hull is

A buckwheat hull is the inedible seed casing left after the groat is milled out, and it reaches a pillow as a by-product of food production rather than as a crop of its own.

What does polishing do?

It tumbles the hulls to knock off dust and adhering fibre, which lowers the scent and the grit and changes how freely the fill shifts.

Unpolished hulls smell more, dust more, and move less. Neither state is better; they suit different sleepers and the pages say which.

Is it the same as the grain?

No. The groat is the food; the hull is the casing around it, and only the casing ends up in a pillow.

Conflating the two is why buckwheat allergy questions get answered badly.