Leviticus 23:22. "Neither shall you gather any gleanings of your harvest"

When the farmer collects his crops, he ties sheaves of stalks together into bundles.

This makes it easier for him to gather the crop afterwards.

All he has to do is collect the bundles.

While he is collecting the stalks, one or two may fall out of his hand or the container, before he ties them into bundles.

The Torah forbids him to pick up those fallen stalks. They are left for the needy to come and collect.