Shabbos, Chapter 3, Mishnah 2. In the previous Mishnah we dealt with the law of a kirah - a type of stove that had two openings on its upper surface, on which pots were placed for cooking. We learned that a kirah heated with straw or stubble does not produce a significant quantity of coals, and therefore there is no concern that one might stir the coals; accordingly, one may place a pot of food on it before Shabbos and let it continue cooking on Shabbos. But if it was heated with wood or the like, one may not leave the pot on it, unless it is gerufah - the coals have been removed from it, or ketumah - ashes have been placed over the coals.
An oven heated with straw and stubble:
Our Mishnah deals with an oven. An oven is a vessel whose primary use is for baking inside it, and its heat is incomparably greater than that of a kirah. For that reason the concern that one might stir the coals is greater, even when only a few coals are present in it. Therefore we learned: "Tanur shehisikuhu b'kash uv'gevava, lo yiten, bein mitocho bein me'al gabav" - even when the heating was done with straw and stubble, both ways are forbidden:
Mitocho - placing something inside the oven to bake.
Me'al gabav - placing a pot on top of the oven.
And not only that, but even gerufah and ketumah are of no help in the case of an oven: even if the remaining coals were removed, or ashes were placed over them, it is still forbidden - and this is unlike the law of a kirah.
A kupach:
The Mishnah goes on to deal with a third vessel, whose heat lies between that of an oven and that of a kirah, and that is the kupach. The kupach is primarily meant for cooking on top of it, like a kirah, except that a kirah is rectangular and has two openings, while a kupach is square and has only a single opening. And from here come its two laws:
"Hisikuhu b'kash uv'gevava - harei hu k'kirah" - just as with a kirah heated with straw there is no concern that one might stir the coals, so too with a kupach, and one may place a pot of food on it before Shabbos and leave it to cook on Shabbos.
"B'gefes uv'eitzim - harei hu k'tanur" - gefes, which is the residue of crushed olives, and likewise wood, produce coals. Once it has been heated with them, its law is that of an oven: even when it is gerufah and ketumah - the coals removed or ashes placed over them - one may not place the food either inside the kupach or on top of it before Shabbos in order that it cook on Shabbos.
In summary: in this Mishnah we learned three levels of heat: the kirah, whose lenient law was explained in the previous Mishnah; the oven, whose heat is great and therefore both placing food inside it and placing food on top of it are forbidden even with straw and stubble, and even gerufah and ketumah do not permit it; and the kupach, whose law is determined by the type of fuel used - with straw and stubble it is like a kirah, and with olive residue and wood it is like an oven.