Before us is Mishnah 6 in the eighth chapter of Maseches Eruvin, which deals with "be'er shebein shtei chatzeiros" - a well of water that serves two courtyards which did not make an eruv with one another, and how each of them can be permitted to draw from it on Shabbos.
The case in the Mishnah:
A well of water located between two courtyards that did not make an eruv together, so that part of it lies beneath one courtyard and part beneath the other. A wall stands between the two courtyards, but the wall passes only above the well and does not descend into it. The halachah in this case is that neither of the courtyards may draw water from the well on Shabbos, because the well is a shared domain and a shared area belonging to both of them, and they did not make an eruv with one another.
The solution - a partition ten tefachim high:
The permission depends on making a partition ten tefachim high. As for where the partition is placed, there are several possibilities:
Above the water line: the partition stands exactly above the surface of the water and rises ten tefachim upward.
Inside the water: the partition is set down below, within the water itself.
In the airspace of the well: even if it is located somewhere in the airspace of the well, far above the water line - that is, close to the mouth of the well only, and even though its ten tefachim end well before the water - it is still a sufficiently significant partition, and it has the power to permit each courtyard to draw from the well separately.
The dispute between Beis Shammai and Beis Hillel:
The Mishnah then quotes Rabban Shimon ben Gamliel, who teaches us that the opinion we saw in the first Tanna is the subject of a dispute between Beis Shammai and Beis Hillel:
Beis Shammai: the partition in the well must be exclusively below the water line, that is, within the water itself.
Beis Hillel: the partition is valid even above the water line, and even far above the water line, as long as it is located within the well itself - and not above where the well begins.
The opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees with everything we have seen until now: there is no need for a partition larger or more significant than the existing wall that stands between the courtyards. The wall between the courtyards, which passes above the well, serves as a partition between the two sides of the well, and that is enough. There is no need for an additional partition and an additional division within the well itself or close to its water - the partition passing above it suffices.
In summary: we learned about a well between two courtyards that did not make an eruv, that drawing from it is forbidden because of their shared ownership of it, and that the permission depends on a partition ten tefachim high - according to the first Tanna, whether within the water, above its surface, or anywhere in the airspace of the well. Rabban Shimon ben Gamliel presents this opinion as a dispute: Beis Shammai require a partition within the water, while Beis Hillel validate one even above the water within the airspace of the well. And Rabbi Yehudah is lenient and holds that the existing wall between the courtyards, which passes above the well, is sufficient.