Sukkah, Chapter 2, Mishnah 9. The Mishnah establishes that throughout all seven days of Sukkos a person must make his sukkah his permanent dwelling and his house his temporary one: the sukkah becomes his primary residence, while the house is regarded as no more than a temporary dwelling.
How is this measured? By where a person keeps his finer vessels:
In the sukkah: all the attractive vessels and utensils are brought into the sukkah, and everything of quality belongs there.
In the house: only the secondary items a person happens to need are left behind, and they are merely incidental.
When does rain exempt a person from the sukkah:
The Mishnah goes on to discuss at what point a person is no longer obligated to remain in the sukkah while rain is falling, and from when he is permitted to leave and remove his belongings. The measure given by the Sages is "mishetisrach hamikpah" - the mikpah is a thick cooked dish, something like a heavy porridge, and once the rain ruins and spoils it, a person may leave the sukkah. This is a sign that the rainfall is significant enough.
The parable of the servant and the master:
The Mishnah offers a parable for what this resembles when a person seeks to fulfill the mitzvah of sukkah and the rain comes down: a servant who came to pour a cup of wine for his master, and the master poured the pitcher over his face, as if to say: "I do not want you to serve me."
This is indeed a sobering thought. There is a discussion as to precisely when such a declaration applies, but a person's feeling is that he wishes to fulfill the mitzvah of sukkah, and the Holy One, blessed be He, who controls the rain, is not allowing him to do so. To a certain degree this is humbling: it seems as though the Holy One, blessed be He, does not want us to fulfill the mitzvah of sukkah, a mitzvah we seek to perform because He commanded it, and He does not want us to serve Him in this way at this time.
In summary: in this Mishnah we learned the obligation to make one's sukkah permanent and one's house temporary, which includes moving the finer vessels into the sukkah and leaving the secondary items in the house; the measure from which one may leave the sukkah when rain falls - "mishetisrach hamikpah"; and the parable of the servant whose master poured the pitcher over his face, from which emerges the sense of distance conveyed by rainfall on the festival.