The Torah teaches (Shmot 12:10) that we are commanded to burn the notar, i.e. any meat that is left over from the korban Pesah. The Mishna on our daf teaches that this notar is not burned on Yom Tov, nor is it burned on Shabbat, so if the first day of Pesah is on a Friday, the notar would not be destroyed until Sunday.
Aside from the edible meat that was left over, the Mishna teaches about two other parts of the animal that need to be burnt – the bones and the gid ha-nasheh.
The bones of the korban Pesah cannot be broken (see Shmot 12:46) so, by definition, there will be edible marrow left within them that will be considered notar and will have to be destroyed.
The gid ha-nasheh is a sinew in the animal’s hind-quarters that is forbidden by the Torah based on Ya’akov’s injury after wrestling with the angel (see Bereshit 32:31-32). Since the gid itself is forbidden, the Gemara concludes that the Mishna is talking about a part of the sinew that is permitted on a Biblical level and is forbidden as a Rabbinic ordinance.
The Sha’agat Aryeh (Rabbi Aryeh Leib Gunzberg) asks why the gid ha-nasheh of the korban Pesah would be forbidden at all. Based on the general principle that aseh doheh lo ta’aseh – that a positive commandment will “push aside” a negative one, shouldn’t the mitzva of eating the korban Pesah override the prohibition of eating the gid ha-nasheh? One suggested answer is that the mitzva forbidding the gid ha-nasheh was given even before the Torah was presented to the Jewish people on Mount Sinai, and therefore its rules are more stringent. The Sefat Emet suggests that this is an example of a case where the Sages chose to give added strength to one of their decrees, even in the face of a Biblical commandment.