The thresher or thrasher machine uses its drums and shakers to separate grains from the stalks and the husks. In other words, it separates the wheat from the chaff.
Threshing was done by hand for thousands of years before the machine was invented. The wheat sheaves were laid on the ground and beaten with flails. The remnants were then thrown into the air from baskets to separate the loosened chaff from the grain. This was slow, hard work. It kept a great number of people in work.
So when the threshing machine was invented and came into use in the 18th century, it was seen as a threat to the livelihoods of the agricultural workers. In Kent and Sussex, and later as far as the Midlands, farm workers set out the destroy the machines. These riots were called the Swing Riots and part of the anti-machinery Luddite movement.
The first threshers were set in the barn. But when a horse or engine could be used to power the machine, it meant that the thresher could be taken into the fields.
There is a film listed on the BFI website of a threshing machine>>
Jean Francois Millet: The Winnower