Peah, Chapter 7, Mishnah 3. This mishnah deals with the mitzvah of peret - leaving the fallen grapes for the poor. And what is peret? As we learned in the previous chapter, peret is essentially similar to leket: leket refers to the stalks that fall during the harvesting of grain, while peret refers to the individual grapes that fall during the grape harvest.
"Eizehu peret?":
The mishnah opens with the question "Eizehu peret?" - what qualifies as peret? And it answers: "Hanosher bish'as habetzirah" - those individual grapes that fall at the time of the grape harvest.
A case that looks like peret but is not:
The mishnah presents a case that one might have thought should be considered peret, but in fact is not: "Hayah botzer, akatz es ha'eshkol, husbach be'alim, nafal miyado la'aretz venifrat - harei hu shel ba'al habayis" - a person is harvesting his vineyard and cuts the stem of the cluster, and the cluster or his hand gets entangled in the large vine leaves; because of that entanglement the cluster falls from his hand to the ground and breaks apart, and individual grapes scatter from it. In this case the grapes do not belong to the poor but to the owner, because they fell due to circumstances beyond his control and against his will, and not in the normal course of the harvest. This is an irregular situation, and therefore it is not considered peret.
It is worth noting that the quantity of grapes considered peret is one grape, or at most two, at a time, since three grapes are already not considered peret - as we learned in the previous chapter.
Placing a basket under the vine:
"Hamaniach es hakalkalah tachas hagefen bish'ah shehu botzer - harei zeh gozel es ha'aniyim" - one who places his basket under the vine at the time of the harvest causes the peret, those grapes that would have fallen to the ground and been considered peret, to fall into his basket instead. In doing so he robs the poor, since he prevents them from receiving the peret that was meant to fall to the ground and remain there.
Of such a person the verse in Mishlei says: "Al taseg gevul olim" - "Do not move back the boundary of those who ascend." 'Olim' means those who go up, and in fact this is a euphemism for those who have gone down, people who lost their property and became poor. The verse uses inverted language here, calling those who have gone down by the name 'olim.' And the meaning of the verse is: do not encroach upon their boundary and upon what belongs to them - and that is exactly what happens when a person places his basket under the vine at the time of the harvest.