Eruvin, Chapter 8, Mishnah 11. This mishnah is a continuation of the two preceding mishnayos, which deal with pouring waste water into a courtyard on Shabbos.
The text of the Mishnah:
"Vechein shtei deyotos zu keneged zu" - deyotos are two-story buildings. The case, then, is of two two-story buildings standing opposite one another, with a shared courtyard between them.
"Miktzasan asu ukah" - one of them made an ukah, the pit described in the previous mishnayos, which permits pouring waste water into the courtyard on Shabbos even if the courtyard is smaller than four amos by four amos.
"U'miktzasan lo asu ukah" - and the other did not make such a pit.
"Es she'asu ukah mutarin, veshelo asu ukah asurin" - those who made the ukah are permitted, and those who did not make one are forbidden.
What exactly is permitted and what is forbidden?
The case under discussion is where the two buildings did not make an eruv chatzeiros between them, and therefore their residents may not carry the water out of their houses into the courtyard. The question is whether they may pour the waste water inside the house in a way that it flows out on its own into the courtyard.
On this the Mishnah rules: those who made an ukah may pour the water inside their house even though it will ultimately run out into the courtyard, since the ukah is relatively close to them and there is no concern that they will carry the water out by hand. But regarding one who did not make an ukah, we are concerned that he will carry the water out into the courtyard in order to bring it to the ukah, and therefore it was forbidden.
The Tosafos Yom Tov explains that the Mishnah deliberately speaks of two-story buildings in order to highlight a point: the concern that they will carry the water out into the courtyard exists even with regard to the residents of the second floor, and even where that much bother is involved, the Sages decreed.
All of this applies when they did not make an eruv chatzeiros. If they did make an eruv chatzeiros, a single ukah holding two se'ah suffices, the measure explained in the mishnah two mishnayos earlier, and all the residents of the courtyard may pour their waste water into that courtyard. And if the water spills over and flows outside, there is no concern, since it does not appear as though it was done intentionally.
In summary: In this mishnah we learned that in the case of two buildings standing opposite one another without an eruv chatzeiros, one who made an ukah may pour waste water inside his house even though it runs out into the courtyard, while one who did not make an ukah may not, out of concern that he will himself carry the water out to the ukah - a concern that applies even to residents of the second floor. And when they did make an eruv chatzeiros, a single ukah holding two se'ah is enough to permit it for all the residents.