Gleaning is the act of collecting leftover crops from farmers' fields after they have been commercially harvested or on fields where it is not economically profitable to harvest. Some ancient cultures promoted gleaning as an early form of a welfare system.
According to the Holiness Code

Holiness code

The Holiness Code is a term used in biblical criticism to refer to Leviticus 17-26, and is so called due to its highly repeated use of the word Holy. It has no special traditional religious significance and traditional Jews and Christians do not regard it as having any distinction from any other...

 and the Deuteronomic Code

Deuteronomic Code

The Deuteronomic Code is the name given by academics to the law code within the Book of Deuteronomy. It contains "a variety of topics including religious ceremonies and ritual purity, civil and criminal law, and the conduct of war"...

 of the Torah

Torah

Torah- A scroll containing the first five books of the BibleThe Torah , is name given by Jews to the first five books of the bible—Genesis , Exodus , Leviticus , Numbers and Deuteronomy Torah- A scroll containing the first five books of the BibleThe Torah , is name given by Jews to the first five...

, farmer

Farmer

A farmer is a person engaged in agriculture, who raises living organisms for food or raw materials, generally including livestock husbandry and growing crops, such as produce and grain...

s should leave the corners of their fields unharvested, and they should not attempt to harvest

Harvest

Harvest is the process of gathering mature crops from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper...

 any left-overs that had been forgotten when they had harvested the majority of a field.