“wheat…chaff.” This is the figure of speech hypocatastasis; the wheat and chaff represent people. The “wheat” was the grains of wheat that could be ground into flour, while the chaff was small broken pieces of the stalk on which the wheat grew. For more on hypocatastasis, see commentary on Revelation 20:2, “dragon.”
“he will thoroughly clean out his threshing floor.” The threshing floor is a metaphor for the final judgment when there will be “wheat” people (the saved) and “chaff” people (the unsaved). The floor will be thoroughly cleaned out—no one will escape the final judgment.
“barn.” This word could also be in reference to a granary which would normally be a pit in the ground. Regardless, it is in reference to a place where grain is stored. In general, the word “barn” is a historic anachronism because the granaries and storehouses used by people of biblical times in the ancient Near East were not like our modern barns but were much more modest storehouses. Archaeology has revealed that in many cases, grain was even stored in large clay storage jars which could be closed with a lid or stone on top.