“they are like the chaff.” The point that this Psalm is making is that there is a Day of Judgment coming, and on that day the unrighteous people, the unsaved people, will be like the chaff after winnowing: they will be gone. To best understand the simile that the wicked are like the chaff that the wind blows away, it is helpful to understand ancient farming practices.
In biblical times the ripe grain was harvested by hand. The grain stalk was cut off close to the ground with a sickle or knife of some kind. When a person had cut too many stalks to easily hold, he or she wrapped them in a bundle (usually by wrapping some of the stalks around the rest) and left them on the ground so they could be easily seen and gathered. Once all the grain in the field had been cut, all the bundles were gathered up and carried to a threshing floor. At that point, the grain was still on the stalk.
The threshing floor was a large flat area of rock or very hard ground. It was usually on top of a hill so the breeze would blow across it. “Threshing” was the process of separating the grains of wheat from the stalk, and various methods were used to do that. The most primitive method was simply to beat the wheat—or whatever grain was being harvested, such as barley or millet—with a stick over and over to knock the grain off the stalk (Judg. 6:11).
Another method of threshing was to have some cows or oxen walk back and forth over the pile of grain. Their hoofs would cut up the wheat stalk and knock the grain off the stalk. As the cattle walked over the grain, it was against the Mosaic Law to keep them from eating it (Deut. 25:4). A more efficient way of threshing was done with a “threshing sled,” which was a flat-bottomed sled with metal or stone teeth set into the bottom. When the ox pulled the sled back and forth over the grain, the teeth on the bottom of the sled would cut up the stalk and also separate the grain from the stalk (Isa. 41:15; Amos 1:3).
After the grain was threshed, there was a large pile of broken stalks, chaff, and grain all mixed together on the threshing floor. The chaff was the very small pieces of stalk, while the broken grain stalks were the larger pieces. To separate the grain from the broken shafts and the chaff, the farmer would wait for a breeze and then winnow the pile with a winnowing fork. The winnowing fork (sometimes called a winnowing shovel), was a tool much like a pitchfork but with flat wooden tines. The farmer would thrust the winnowing fork into the pile and toss it as high as he could into the air. The breeze would catch the wheat shafts and chaff, which were very light, and blow them to the side. The grains of wheat, which were heavier and oval-shaped, would fall almost straight back down to the threshing floor.
After a number of hours of winnowing, the wheat would be left on the threshing floor, the larger pieces of the shafts would be blown off to the side not too far away, and the much smaller chaff would be blown even further away or blown away completely. At that point, the grain could be taken to the granary or storehouse, while the wheat shafts and chaff were used in other ways. Sometimes the large pieces of the shaft were used in making bricks, which is why the Bible speaks of making bricks with “straw,” which was the grain shafts (Exod. 5:7-18). Sometimes the shafts and chaff were burned as fuel in the ovens because it burned quickly and very hot, and it heated up the clay ovens very rapidly (Matt. 3:12).
Sometimes, however, the winnowing breeze was hard enough that the chaff simply blew away and could not be found. That is the illustration in this Psalm: on Judgment Day the wicked will not be left standing in the assembly of the righteous. They will be gone like the chaff in the wind. Therefore, chaff is a good illustration for the unsaved people because just like the chaff is often taken and burned up, the unrighteous chaff-people will be thrown into the Lake of Fire and will be consumed there.
[For more on the harvesting and sifting of grain, see commentary on Amos 9:9. For more on annihilation in the Lake of Fire, see Appendix 4: “Annihilation in the Lake of Fire. There are many good books on biblical manners and customs that describe the processes used in harvesting grain.]