Orlah, Chapter Two, Mishnah 16. In the previous Mishnah we dealt with Terumah and kilei hakerem (forbidden mixed growths of a vineyard) that combine together, and with the question of whether their combination creates a prohibition, a matter in which the first Tanna and Rabbi Shimon disagreed. The interesting point there was that the Terumah is permitted to a kohen: since the Terumah alone is not enough to create a prohibition, and the kilei hakerem alone are not enough to create a prohibition, but only the two together - for a kohen it is permitted, and for a non-kohen it is forbidden.
Our Mishnah is similar in concept to the previous one, and deals with sacrifices: a piece of kodshei kodashim (offerings of the highest sanctity) that was cooked together with other pieces.
Introduction - kodshei kodashim and kodashim kalim:
Kodshei kodashim - those of them that are eaten are eaten by kohanim only.
Kodashim kalim (offerings of lesser sanctity) - eaten by any Jew, as long as he is tahor and not tamei.
Piggul and nosar:
Piggul - when the kohen, while performing one of the sacrificial services, intended to perform one of the acts of the offering, or to eat from it, outside of its proper time. This intention renders the meat of the offering itself piggul, and it is forbidden to be eaten.
Nosar - meat of an offering that remained beyond the time in which it may properly be eaten, and once that time has passed it may no longer be eaten by anyone.
The basic difference between them: kodshei kodashim are permitted to kohanim and forbidden to a non-kohen, whereas piggul and nosar are forbidden to everyone, including the kohanim themselves.
The case in the Mishnah:
"Chatichah shel kodshei kodashim v'shel piggul v'shel nosar shenisbashlu im hachatichos" - a piece of meat of kodshei kodashim, which is eaten by kohanim only, and a piece of meat of piggul and of nosar, which is forbidden to everyone, were cooked together with other pieces of chullin (ordinary food), which are not sacrificial at all. The case is one in which the taste of each piece on its own is not enough to be discernible in the permitted pieces, but only the two of them combined together.
"Asur l'zar umutar lakohanim":
For a zar, one who is not a kohen, the mixture is forbidden: the zar is forbidden to eat kodshei kodashim and is forbidden to eat piggul and nosar, and since both prohibitions apply to him equally, they combine together to reach the amount that is discernible in the pieces of chullin and render them forbidden.
For a kohen, by contrast, the pieces of chullin are permitted: the only thing forbidden to him is the piggul and nosar, and their taste alone is not enough to be discernible in the pieces of the mixture. The kodshei kodashim are permitted to him, and therefore they cannot combine to complete the measure of prohibition, and the kohen may eat.
The view of Rabbi Shimon:
"Rabbi Shimon matir l'zar v'lakohanim" - Rabbi Shimon permits the mixture not only to a kohen but even to a zar, even though the zar is forbidden both in kodshei kodashim and in piggul and nosar. His view is that two different prohibitions do not combine together to create a prohibition.
In summary: This Mishnah continues the principle of the previous Mishnah, now in matters of sacrifices. A piece of kodshei kodashim and a piece of piggul and nosar that were cooked with pieces of chullin, where the taste of each one alone is not enough to be discernible - according to the first Tanna they combine to forbid the mixture to a zar, since both are forbidden to him, while for a kohen it is permitted, because the kodshei kodashim are permitted to him and the piggul and nosar alone do not amount to a discernible measure. Rabbi Shimon permits it even to a zar, consistent with his view that two different prohibitions do not combine with one another.