The second mishnah in the second chapter of Maseches Sukkah deals with several ways of setting up a sukkah and laying the sechach, and with the question of when the sukkah is valid and when it is invalid.
"HaSomech sukkaso b'kar'ei hamitah":
One who supports the walls of his sukkah on the legs of a bed, in such a way that moving the bed would drag the entire sukkah along with it, and might even cause it to collapse together with the sechach: the sukkah is valid.
Rabbi Yehudah says: "Im einah yecholah la'amod bifnei atzmah - pesulah" - since the sukkah does not stand without the legs of the bed, and moving the bed causes it to fall, it is invalid. Rabbi Yehudah is following his own approach here, that a sukkah must be a permanent dwelling, and since this is not a permanent structure, it is invalid.
"Sukkah hamedubleles":
A sukkah whose sechach is jumbled, meaning that it is not laid out in a neat and orderly fashion, but rather one branch is raised upward and another is lower down, so that more sun than shade comes through it. Even so, if an orderly arrangement of the branches would have provided enough sechach for the shade to exceed the sunlight, and as long as the branches are within three tefachim of one another, it is valid.
"V'shetzilasah merubah mechamasah" - even a sukkah whose sechach is very sparse and thin is valid, as long as its shade exceeds its sunlight. Both of these cases are valid.
Some have a version of the mishnah that reads the two as a single case: "Sukkah hamedubleles shetzilasah merubah mechamasah" - a sukkah whose sechach is jumbled, and nevertheless its shade exceeds its sunlight, and it is therefore valid.
"Me'ubah k'min bayis":
Sechach that is extremely thick, "af al pi she'ein hakochavim nir'in mitochah" - so that the stars cannot be seen through it - is valid.
Nevertheless, the wording of the mishnah indicates that l'chatchilah it is proper that one be able to see the stars through the sechach, and only b'dieved is the sukkah valid even when the sechach is so thick that the stars are not visible through it.
In summary: in this mishnah we learned three rulings: one who supports his sukkah on the legs of a bed - it is valid, while according to Rabbi Yehudah, who holds that a sukkah must be a permanent dwelling, it is invalid if it cannot stand on its own; a jumbled sukkah and a sukkah with sparse sechach are valid, provided that the shade exceeds the sunlight; and sechach that is as thick as a house is valid b'dieved, even though l'chatchilah it is proper that the stars be visible through it.