In the previous Mishnah we learned about an enactment of beis din: a person who finds an animal, and once it has been determined for which offering it is fit to be brought, the libations of that offering are brought from the public funds - from the terumas halishkah (the chamber donation) - and not from the finder himself.
To this Rabbi Shimon responds: "Shivah devarim hiskinu beis din" - beis din instituted seven enactments, particularly in matters of libations and meal offerings and their related laws - "vezeh echad mehen" - and this is one of them. That is, the halachah of the previous Mishnah, that the libations of an animal that was found and determined fit for a particular offering are brought from the terumas halishkah, is the first of the seven enactments listed here. From here we continue to enumerate the rest.
A non-Jew who sent his olah from overseas:
"Nachri sheshilach olaso mimedinas hayam" - a non-Jew who sends his olah (burnt offering) from overseas, "veshilach imah nesachim" - and sent libations along with it. This does not mean the libations themselves, that is, actual wine, fine flour and oil, since those would be tamei, but rather the money for the libations that was sent along with the offering. If he sent the money along with it - "kereivim mishelo" - and the libations are brought from those funds. "Ve'im lav" - if for some reason he sent only the olah and did not send libations - "kereivim mishel tzibbur" - they are brought from the public, and this is the enactment of beis din, that the libations be brought from the funds of the terumas halishkah.
A convert who died and left offerings:
The next enactment: "Vechein ger shemeis" - a convert who died, and who presumably died without heirs, "vehiniach zevachim" - and left behind animals that had already been consecrated as offerings. Here too there are no heirs to bring the libations on his behalf. If he set aside libations together with the offerings while he was still alive - "kereivin mishelo" - the libations are brought from what he set aside. "Ve'im lav" - "kereivin mishel tzibbur" - and the enactment of beis din is that they be brought from the public, from the terumas halishkah.
The minchas chavitin of a Kohen Gadol who died:
The next case deals with the minchas chavitin, the meal offering that the Kohen Gadol brought every day - half in the morning and half in the afternoon. The Mishnah discusses a Kohen Gadol who died after bringing the morning half and before bringing the afternoon half, and teaches that beis din made a stipulation: the second half is brought from the public.
Tanna Kamma (Rabbi Shimon): It is a stipulation of beis din that the second half be brought from the public.
Rabbi Yehudah: He disagrees and says that the heirs are the ones who bring the second half.
But all agree with the final ruling: in this case one does not bring only an additional half, but rather the entire minchas chavitin. One does not follow the usual practice of half in the morning and half in the afternoon; rather, whatever would ordinarily be offered in the morning and in the afternoon is all brought together in the afternoon as a completion - whether it is brought by the heirs according to Rabbi Yehudah or by the public according to the Tanna Kamma. This law is derived from the verses.
The Gemara's explanation - restoring the halachah to its place:
The Gemara explains that according to the Tanna Kamma, who is Rabbi Shimon, who says that there was a stipulation of beis din here, beis din in fact restored the halachah to its original place. For by Torah law this meal offering is to be brought from the public, and this is the original halachah derived from the verse "chok olam" - from the world, from the public. The only problem was that in situations where a long time passed until another Kohen Gadol was selected and appointed in his place, it became difficult for the chamber to bear the expense, and therefore they enacted that the heirs bring the meal offering. However, when they saw that the heirs were negligent about it, they returned the matter to the original Torah law. This is the enactment of beis din to which our Mishnah refers: that the meal offering be brought from the public, as it was by the essential law.
In summary: In this Mishnah Rabbi Shimon enumerated the seven enactments that beis din instituted in matters of libations, and we learned the following from them: the enactment concerning the libations of an animal that was found, which are brought from the terumas halishkah; a non-Jew who sent his olah from overseas - if he sent libation funds along with it they are brought from his own, and if not, from the public; a convert who died and left offerings - if he left libations they are brought from his own, and if not, from the public; and the minchas chavitin of a Kohen Gadol who died after bringing the morning half, which according to the Tanna Kamma is brought from the public and according to Rabbi Yehudah from the heirs, and according to all opinions is brought in its entirety at one time. We also saw that according to Rabbi Shimon beis din did not introduce a new law, but rather restored the halachah to its essential Torah source.