Threshing is the process of loosening the edible part of cereal grain from the scaly, inedible chaff that surrounds it. It is the step in grain preparation before winnowing, which separates the loosened chaff from the grain. Threshing does not remove the bran from the grain.
Threshing may be done by beating the grain using a flail on a threshing floor. However, in developed areas it is now mostly done by a threshing machine, now usually by a combine harvester, which threshes as as well as harvesting the plant and cleaning the grain). The threshing machine which preceded the use of combines, was usually driven by a steam engine, or later by a tractor. They were moved from farm to farm & set up next the stack of corn sheaths.
Another traditional method of threshing is to make donkeys or oxen walk in circles on the grain on a hard surface. (A modern version of this in some areas is to spread the grain on the surface of a country road so the grain may be threshed by the wheels of passing vehicles - sound a bit odd any ref to this ?).