Jamieson, Fausset and Brown on Deuteronomy 24:19-22

19-22. When thou cuttest down thine harvest in thy field--The grain, pulled up by the roots or cut down with a sickle, was laid in loose sheaves; the fruit of the olive was obtained by striking the branches with long poles; and the grape clusters, severed by a hook, were gathered in the hands of the vintager. Here is a beneficent provision for the poor. Every forgotten sheaf in the harvest-field was to lie; the olive tree was not to be beaten a second time; nor were grapes to be gathered, in order that, in collecting what remained, the hearts of the stranger, the fatherless, and the widow might be gladdened by the bounty of Providence.