Husks of wheat or other cereal. It was separated from the grain by threshing, whereupon it usually blew away, as it was lighter than the grain. Sometimes however, it was burned as fuel. The wicked are frequently likened to chaff in Scripture.
Grain was threshed on outdoor threshing-floors to separate the chaff
Characteristics of chaff
It is blown by the wind Job 13:25
It is light Job 41:28
It is useless Isa 33:11
The wicked are compared to chaff