Sefer HaChinuch ספר החינוך

To not gather the peret of the vineyard: To not gather the peret of the vineyard, but [rather] to leave them over to the poor, as it is stated (Leviticus 19:10), “and the peret of your vineyard, you shall not gather.” And that is the berries that shed at the time of the reaping of the grapes. And the law is the same for other trees that are similar to a vineyard — that we are obligated not to gather the berries that shed.

A hint of the roots of the commandment is like the matter that I wrote above about the corner (Sefer HaChinukh 216).

From the laws of the commandment is that which they, may their memory be blessed, said (Mishnah Peah 6:5), “Which is fallen grapes? That is one or two [grapes]” that separate from the cluster at the time of the grape harvest. “But three [grapes]” that fell at one time “are not fallen grapes.” If he was harvesting and he threw them to the ground, when he removes the clusters, even half of a cluster that is found [all separated] there is fallen grapes; and so [too,] a whole cluster that separated there, behold it is fallen grapes. And [regarding] the one who places his basket under the vine at the time of the grape harvest — behold, this one robs the poor (Mishnah Peah 7:5). [These] and the rest of its details are in Tractate Peah. (See Tur, Yoreh Deah 332.)

And the content of the commandment, in which place it is practiced and by who — is [all] written above in the commandment of the corner (Sefer HaChinukh 216, 220).