What is Chaff?

Chaff is the straw which is collected along with the grain when such crops
as wheat, barley, millet and rice are harvested. In biblical times, the
wheat was removed from the chaff by throwing both up into the air. The
wheat, being more dense, would fall back into the original container,
while the chaff, being much less dense would blow away into the wind. This
properties of chaff–that it is basically worthless and that it is
separated by throwing the harvested materials into the air explains the
allegorical use it was put to in several biblical passages.

John Oakes, PhD