Maseches Sukkah opens with Chapter 1, Mishnah 1. The first two chapters deal with the laws of the sukkah itself, while the rest of the tractate is divided as follows:
Chapters 1 - 2: the laws of the sukkah itself.
Chapter 3: the laws of the four species - esrog, lulav, hadassim and aravos.
Chapters 4 - 5: the laws relating to the days of Sukkos in the Beis HaMikdash.
"Sukkah shehi gevohah lemalah me'esrim amah - pesulah":
A sukkah higher than twenty amos is invalid. The reasoning of the Tanna Kamma is that a sukkah must be a temporary structure and not a permanent one, and a structure taller than twenty amos requires sturdier construction, to the point that it becomes a permanent building - and therefore it is invalid.
Rabbi Yehudah, in contrast, validates a sukkah even if it is higher than twenty amos, since in his view "sukkah diras keva ba'inan" - a sukkah is required to be a permanent structure. For that reason it is valid even at such a height, since by its very definition it must be built as a permanent structure.
The other disqualifications in the Mishnah:
"Vesheynah gevohah asarah tefachim" - this is the minimum measure; a sukkah that is not ten tefachim high is invalid.
"Vesheyn lah shalosh dfanos" - a sukkah that does not have three walls is invalid.
"Veshechamasah merubah metzilasah" - if the sunlight coming in exceeds the shade created by the s'chach, it is invalid.
The word "pesulah" in the Mishnah therefore applies to all of these cases: a height of less than ten tefachim, the absence of three complete walls, and sunlight exceeding shade.
Three walls - how so?
The Gemara explains that there are situations in which the sukkah is valid even with fewer than three complete walls, and this is derived from a halachah leMoshe miSinai:
Two perpendicular walls: one takes a board slightly wider than a tefach and stands it within three tefachim of one of the walls. The tefach plus the additional bit, together with the gap of less than three tefachim - which is considered closed by the law of lavud - combine to four tefachim. Since the measure of a sukkah wall is seven tefachim, four tefachim constitute the majority of a wall, and that is enough for it to be considered three walls.
Two parallel walls: one takes a board slightly wider than four tefachim and stands it within three tefachim of one of the walls. The four tefachim and a bit more, combined with the gap of less than three tefachim by the law of lavud, create a complete wall of seven tefachim.
In both of these cases a tzuras hapesach is required in the open space, and only when it is made is the sukkah valid.
"Sukkah yeshanah":
A sukkah that was already standing beforehand, and was not made for the sake of a sukkah but for another purpose - such as a shaded spot to escape the sun during the summer months. Beis Shammai invalidate it, since in their view the sukkah must be made for the sake of the mitzvah of sukkah; and Beis Hillel validate it, for it is enough that it was made for shade and it need not be made for the sake of the mitzvah.
"Ve'eizo hi sukkah yeshanah? Kol she'asa'ah kodem lechag sheloshim yom" - one who makes his sukkah more than thirty days before the festival is assumed not to have made it for the sake of the festival but merely for shade, and it is about this that Beis Shammai and Beis Hillel disagree.
"Ve'im asa'ah leshem chag, afilu mitechilas hashanah - kesheirah" - even if he made it many months before Sukkos, since he made it for the sake of the mitzvah of sukkah, it is valid.
In summary: in this Mishnah we learned the disqualifications of a sukkah - a height above twenty amos (according to the Tanna Kamma, who holds that a sukkah must be a temporary dwelling, as opposed to Rabbi Yehudah who validates it), a height of less than ten tefachim, the absence of three walls, and sunlight exceeding shade. We examined the law of lavud and tzuras hapesach with two perpendicular walls and with two parallel walls, and the dispute between Beis Shammai and Beis Hillel regarding an old sukkah - a sukkah made more than thirty days before the festival and not for the sake of the mitzvah.