The requirement of “Peret” obligates the owner of a vineyard to leave for the poor grapes which fall to the ground during the harvest. The Torah in Parashat Kedoshim (Vayikra 19:10) formulates this Misva as both an affirmative command, requiring one to leave these grapes for the poor, and a prohibition, forbidding taking these grapes for oneself.
Halacha defines “Peret” as one or two grapes which fell to the ground during the harvest. If, however, three or more grapes fall together, then they are not considered “Peret,” and the vineyard’s owner may keep them for himself. This does not apply if the harvester knocked all the grapes to the ground, and then proceeded to collect them from the ground and place them in a basket. In such a case, no matter how many grapes were left on the ground after the harvester left, they are all considered “Peret” and belong to the poor.
If one places a basket underneath the vine as he harvests to collect the grapes that fall, he is considered a thief, as he steals the “Peret” from the poor.
The prohibition against keeping the “Peret” for oneself falls under the category of “Lav Ha’nitak La’aseh” – a prohibition which can be rectified, and for which violators are thus not liable to Malkut. If one transgressed this prohibition, he corrects his mistake by transferring the grapes in question to the poor, and so he is not liable to Malkut.