Pe'ah, Chapter 2, Mishnah 3. The two previous mishnayos listed various items that are considered a divider between two parts of a field, and that have the power to make the two parts into two separate fields with regard to pe'ah. Our mishnah opens with a broad rule:
"Hakol mafsik lezera'im":
All the items listed in the two previous mishnayos serve as a divider and a separation for seeds, that is, for the plants growing in the field. In a field where grain is growing, or various vegetables and the like, those items form a divider in it and make its two parts into two separate fields with regard to pe'ah.
"V'ein mafsik la'ilan ela gader":
But when we are dealing with an orchard - where there are trees in the field and there is an obligation to give pe'ah from the fruit of the trees as well - nothing divides except a fence alone. An actual fence separating between them is required, and anything less than that is not considered a divider, and the whole area is treated as one orchard and one field. Even if a stream runs through it, or a private path or a public road passes through it - not one of the items listed earlier is considered a separation, and it is all one orchard with regard to pe'ah, unless there is an actual fence.
"V'im hayah se'ar koshesh - eino mafsik, ela notnin pe'ah lakol":
"Se'ar" ordinarily means hair, but in this context it refers to branches that intertwine and hang down over the fence. In such a case even the fence itself is not considered a divider, and one is obligated to give pe'ah from the entire orchard: even the portion on the other side of the fence is considered one field, and only a single pe'ah must be given from that field, and not two separate ones.
The addition of the Yerushalmi:
The Yerushalmi notes that the expression "Hakol mafsik lezera'im" is an inclusive term, and comes to include something that was not mentioned in the previous mishnayos with regard to seeds: a rock that runs through the middle of the field. If the one plowing had to lift the plow in order to pass over the rock and could not plow through it, the rock is considered a divider with regard to seeds.
In summary: In this mishnah we learned that all the items listed above divide with regard to seeds, whereas for trees nothing divides except an actual fence; and even that fence ceases to be a divider when branches hang down over it, in which case one gives a single pe'ah for the whole. We also learned from the Yerushalmi that the term "hakol" includes even a rock that the plow cannot pass through.