'Threshing' in the Bible

Now Ornan turned back and saw the angel, and his four sons who were with him hid themselves. And Ornan was threshing wheat.

Ornan said to David, “Take it for yourself; and let my lord the king do what is good in his sight. See, I will give the oxen for burnt offerings and the threshing sledges for wood and the wheat for the grain offering; I will give it all.”