Beitzah, Chapter 5, Mishnah 1. This mishnah revolves around the theme of exertion: the Sages did not want a person to trouble himself excessively on Yom Tov, since it is a time set aside for rest, and for that reason they prohibited many strenuous activities.
The mishnah discusses a person whose produce is laid out to dry on the roof of his house, and he senses that a storm is approaching and rain is about to fall. Since he does not want the produce to be ruined, he wants to bring it back into the house. The case is one where there is an arubah - an opening in the roof - through which the produce can easily be dropped down into the house.
"Mashilin peiros derech arubah b'Yom Tov, aval lo b'Shabbos":
One may drop the produce down through the arubah, the skylight in the roof, on Yom Tov. On Yom Tov the Sages were lenient, but not on Shabbos.
What is unique about the arubah is that dropping produce through it is easier than moving it through a window. Had the entry into the house been through a window set in the wall adjoining the roof, the person would have had to lift the produce and bring it in through the window, whereas toward the arubah it is enough to push it along. For this reason the mishnah permitted only this method, since bringing it in through a window involves more exertion than is permitted.
"U'mechasin peiros b'keilim mipnei ha'dalef":
Here too the discussion is about protection from the rain: produce is lying inside the house, and a leak from the roof is dripping onto it. One may cover it with vessels, and this does not constitute excessive exertion. Moreover, even if the produce were tevel and therefore muktzeh, one may move a vessel in order to protect something that is muktzeh.
"V'chein klei yayin u'klei shemen" - the same law applies to barrels of wine and barrels of oil, when he does not want rain falling into them, and even if they are tevel.
"V'nosnin kli tachas ha'dalef b'Shabbos":
One may place a vessel beneath the leak in order to collect the rainwater dripping into the house, and this is permitted even on Shabbos and not only on Yom Tov. In the other cases the mishnah dealt with what is permitted on Yom Tov, whereas here the permission extends to Shabbos as well. One may also empty out the water when the vessel fills up, provided that the water is fit for use - meaning that it is fit to drink. Clear water such as this may be drunk or even used for washing.
Water that is not fit for use, however, not only may it not be exchanged and emptied out, but it is even forbidden to place the vessel beneath the leak on Shabbos in the first place. We already learned this halachah in Maseches Shabbos: one may not allow something muktzeh to enter a vessel on Shabbos in a manner that will make it impossible to move the vessel afterward. This is like fixing the vessel in its place, and the Sages forbade it by rabbinic decree.