Shabbos, Chapter 1, Mishnah 11. In the previous mishnah we dealt with the concern that a person might stir the coals - when one bakes bread or places a cake on top of coals, there is a concern that he will stir them so that the food cooks faster. Our mishnah discusses cases in which no such concern exists.
Lowering the Pesach offering into the oven:
The halachah is that on Erev Pesach, at the time the Pesach offering is roasted, one may "leshalshel es haPesach batanur" - lower the lamb on its spit from the top of the oven down into it. And one may do this "im chasheichah" - even right at nightfall, immediately before Shabbos begins, and even when Erev Pesach falls on Shabbos. As long as the meat was placed in the oven before Shabbos, there is no concern about its cooking on Shabbos, since the only concern is that he might want to stir the coals in order to speed up the cooking.
The reason is that regarding everything connected with the Seder night and its preparations, a special halachah was stated: those occupied with it "zerizim heim" - they are alert, they are meticulous in keeping the halachah, and there is no concern that they will do something improper.
The fire of the Beis HaMoked:
For this same reason we learned that one may "ma'achizin es ha'ur bemeduras beis hamoked" - kindle the fire in the bonfire that burned in the Beis HaMoked, that place in the Mikdash where a fire burned so that the kohanim could warm themselves. It may be kindled right before Shabbos, even though it continues to burn on Shabbos and it is possible that they will want or need to stir its coals, because the kohanim are alert and are exceedingly meticulous in halachah.
And in the outlying areas:
Outside the Mikdash, in the outlying areas, the concern does exist. Therefore one must make sure that the fire has reached a level of burning at which there is no longer any concern that he will stir its coals, and the measure for this is "kedei shetochaz ha'ur beruban" - enough that the fire takes hold of most of them. Once the fire has taken hold of most of the bonfire, it may be kindled before Shabbos and left to continue burning on Shabbos.
The view of Rabbi Yehudah:
Rabbi Yehudah says: "bapecha, min - kol shehu" - with charcoal, any amount at all. When dealing with coals, a small amount of kindling is enough, since he will have no need to stir them. These types of charcoal were such that once they were lit even a little at the outset, they continue to burn properly without stirring, and therefore there is no concern that he will stir the coals.
In summary: This mishnah lists the cases in which there is no concern that one might stir the coals: lowering the Pesach offering into the oven at nightfall, and kindling the fire in the bonfire of the Beis HaMoked - because those engaged in these tasks are alert and meticulous; and in the outlying areas, where the concern does exist - enough that the fire takes hold of most of the bonfire. And according to Rabbi Yehudah, with charcoal any amount at all suffices, since there is no need to stir it.