Before us is Mishnah 9 in the first chapter of Maseches Sukkah, which deals with the required measurements of the walls and the manner in which they are made: how large a gap is permitted between the wall and the ground, how large a gap is permitted between the wall and the sechach, and what the rule is that allows us to view a partition as though it extends beyond where it actually stands.
Walls made from the top downward:
The Mishnah opens: "HaMeshalshel defanos milma'alah lematah" - a person who builds the walls of his sukkah starting from the upper portion, near the sechach, and weaves the material of the walls downward, and the walls do not reach the ground. The law is: if the wall is "gavo'ah min ha'aretz shelosha tefachim" - three handbreadths above the ground - or more, the sukkah is invalid.
Walls made from the bottom upward:
But if he made the walls in the opposite direction - "milmatah lema'alah" - beginning near the ground and building the wall upward, and it does not reach the sechach: as long as the wall is "gavo'ah min ha'aretz asarah tefachim" - ten handbreadths high, meaning it has the measure of ten handbreadths - the sukkah is valid.
'Gud asik' and 'gud achis':
What is the basis for the difference between the two cases? We have two concepts of extending a partition:
'Gud asik' - we view the partition as though it extends upward from where it stands. This rule we do apply, and therefore a wall that has ten handbreadths is valid even though it does not reach the sechach.
'Gud achis' - that we would view the partition as though it extends and descends downward. This rule we do not apply in this situation, and therefore a wall that is three handbreadths away from the ground is invalid.
The opinion of Rabbi Yosi:
All of the above is the opinion of the Tanna Kamma, and this is how we rule. Rabbi Yosi, however, disagrees and holds that we apply the rule in both directions alike: "Keshem shemilmatah lema'alah asarah tefachim, kach milma'alah lematah asarah tefachim" - just as when building from the bottom upward ten handbreadths suffice, so too when building from the top downward ten handbreadths suffice. Just as when one begins from below and goes upward, ten handbreadths of partition are enough and we view it as extending, even though it does not reach the sechach, so too when one begins from above and goes downward: as long as the wall has ten handbreadths, it is valid, even if it is more than three handbreadths above the ground.
Distancing the sechach from the walls:
The Mishnah adds a further measurement regarding the relationship between the walls and the sechach: if he distanced the sechach three handbreadths from the walls, the sukkah is invalid. The walls must be within three handbreadths of the sechach.
In summary: In this Mishnah we learned that a wall made from the top downward is invalid when it is three handbreadths away from the ground, whereas a wall made from the bottom upward is valid with a measure of ten handbreadths even though it does not reach the sechach - because we say 'gud asik' and we do not say 'gud achis'. Rabbi Yosi equates the two directions and validates ten handbreadths even from the top downward, and the halachah follows the Tanna Kamma. We also learned that distancing the sechach three handbreadths from the walls invalidates the sukkah.