Pe'ah, Chapter 6, Mishnah 5. This mishnah deals with measures: how much may be forgotten and still be considered shichechah (forgotten sheaves), and what quantity is no longer considered shichechah.
The measures of shichechah:
"Shnei omarin - shichechah, u'sheloshah einan shichechah" - two sheaves of grain left forgotten in the field are considered shichechah; three sheaves are not shichechah.
"Shnei tzibburei zeisim v'charuvin - shichechah, u'sheloshah einan shichechah" - two piles of olives or of carobs are considered shichechah; three piles are not shichechah.
"Shnei chotznei pishtan - shichechah, u'sheloshah einan shichechah" - two stalks of flax are considered shichechah; three are not shichechah.
Some explain the reason: once there are three, the quantity is no longer called a sheaf, and the Torah wrote "v'shachachta omer ba'sadeh" - "and you forgot a sheaf in the field." Such a quantity is already considered a stack, a heap of grain, and not a sheaf.
Regarding flax, the commentators explain that the mishnah is speaking of a case where the flax was left to grow until it produced seeds, since flax seeds are fit for eating. Flax itself is not a food, and the laws of leket, shichechah and Pe'ah apply only to things that are fit for eating.
From shichechah to peret:
From here the mishnah moves from the law of shichechah to the law of peret. Peret is essentially identical to leket: leket is what falls during the harvesting of grain, while peret is what falls during the picking of grapes - the very same law, only that it is stated with regard to grapes.
"Shnei gargerim - peret, u'sheloshah einan peret" - two grapes that fall are peret and belong to the poor; three grapes are not peret, and the owner may pick them up and return them to his basket.
"Shtei shibolim - leket, v'shalosh einan leket" - two stalks of grain that fall during the harvest are leket; three are not leket.
The dispute between Beis Shammai and Beis Hillel:
"Elu k'divrei Beis Shammai" - the entire distinction between two and three explained until now is the opinion of Beis Shammai. "V'al kulan Beis Hillel omrim: sheloshah la'aniyim v'arba'ah l'ba'al habayis" - in all of these cases Beis Hillel disagree and hold that three - whether stalks of grain, grapes or sheaves - belong to the poor and have already entered their possession, while four belong to the owner of the field.
In summary: the mishnah established measures in the laws of shichechah, peret and leket - according to Beis Shammai, two for the poor and three for the owner, and according to Beis Hillel, three for the poor and four for the owner.