Beitzah, Chapter 4, Mishnah 2. Our Mishnah discusses which wood may be taken for burning on Yom Tov, and from where it may be brought.
"Ein notlin etzim min hasukkah":
A person may not take wood from the structure of the sukkah. It should be stressed that this does not refer specifically to a sukkah of the festival of Sukkos; this law applies on Pesach or Shavuos as well, and the reference is to a hut. The reason for the prohibition: taking wood that is part of the body of the structure is considered demolishing - breaking down a building or dismantling a tent.
One may take only something adjacent to the sukkah that was not part of it - such as a piece of wood leaning against it from the outside - but not something that was an integral part of the sukkah itself.
From where may wood be brought on Yom Tov:
We are speaking of wood that is no longer attached to the ground, and the Mishnah distinguishes between two places:
From the field: One may bring only from an area where the wood was gathered together before Yom Tov, that is, from what is collected - lying in a pile.
From the karpef: A karpef is not a field, but a large open area that they had outside the city, surrounded by a fence and designated for storing things such as wood. Since the wood in it is designated for use from the outset, one may take from it to burn on Yom Tov even when it is scattered about inside.
However, the Gemara teaches that our Mishnah reflects an individual opinion, and this is not the halachah. In practice: from the field one may not take wood on Yom Tov even from a collected area, while from the karpef one may take only from what is collected - from an area where the wood was gathered together.
What is a karpef?
Rabbi Yehudah: Any enclosure that is near the city, that is, within seventy amos of the city. It should be understood that although Rabbi Yehudah does not say so explicitly, he too holds that it must be a locked area.
Rabbi Yosei: "Kol shenichnasin lo befosachas" - any enclosure that one needs a key to enter, "and even within the Shabbos boundary." In his view the karpef need not be so close to the city and need not be within seventy amos; it is enough that it is within the Shabbos boundary, which is two thousand amos, provided that the place has a lock.
In summary: We learned that one may not take wood from the body of the sukkah because of demolishing, but only from what is adjacent to it; that from the field one takes only from what is collected and from the karpef even from what is scattered, while in practice - from the field not at all, and from the karpef only from what is collected; and that in defining a karpef, Rabbi Yehudah requires proximity of seventy amos to the city, while Rabbi Yosei suffices with an area that is locked with a key, even within the Shabbos boundary.