Eduyos chapter 4, mishnah 4. Our mishnah continues the same subject we saw in the sixth chapter of Maseches Peah - it parallels Peah chapter 6, mishnah 2 - and deals with the laws of shichechah (forgotten sheaves).
The operating principle here is that something a person is going to remember is not considered forgotten. It follows from this that one who leaves an object in a particular place, in a specific spot, will end up remembering what he put there. The question our mishnah discusses is how specific that place has to be.
The text of the mishnah:
"Ha'omer shehu samuch la'gefah v'la'gadish v'la'bakar v'la'kelim u'shecheicho" - the sheaf that is next to a gefah, a stack, cattle, or vessels, and he forgot it. The mishnah lists four locations here:
"La'gefah" - according to most authorities, though not all, this is a stone wall whose stones are not set with mortar. The Rambam explained that it refers to the entrance gate of a garden.
"V'la'gadish" - the pile of grain, alongside which the sheaf was left.
"V'la'bakar" - the farm animals. Some found this difficult, since cattle move from place to place and do not constitute a fixed location, and therefore they said that the vocalization is beker - meaning the cattle shed, or boker - meaning the herdsman.
"V'la'kelim" - the farming implements themselves, such as the sickle and the plow. Others explained that it refers to the tool shed, for the same reason: the implements themselves are not a permanent structure.
In all these cases, since the sheaf was left next to those locations and was forgotten, the Tannaim disagree:
"Beis Shammai omrim eino shichechah" - Beis Shammai say it is not forgotten. Even though it was in fact forgotten, this is not shichechah, since the owner of the field will come back and remember the things he left next to the tool shed or the fence and the like.
"U'Beis Hillel omrim shichechah" - Beis Hillel say it is forgotten. This is not considered a specific place, only next to a specific place, and it is entirely possible that it will be forgotten. Therefore he may not go back and collect it, and the prohibition of shichechah applies to it.
The halachah follows Beis Hillel, and in this case they are the stringent ones: it is forbidden to go back and take the sheaf, because of the prohibition of shichechah.
In summary: This mishnah clarifies the definition of a 'specific place' in the laws of shichechah. A sheaf left next to a gefah, a grain stack, cattle, or vessels - according to Beis Shammai it is not shichechah, since the owner will remember it; and according to Beis Hillel, proximity to a specific place is not the same as the specific place itself, and it is shichechah. As stated, the halachah follows Beis Hillel, who are the stringent ones in this matter.