According to the Mishnah (22b), if someone takes an oath not to eat or drink, and then he ate or drank things that are not edible, he would not be held liable for his shevu’ah. If, on the other hand, he ate and drank foods that are forbidden, like nevelot, terefot, shekatzim u’remasim – animals that were not slaughtered properly or creepy crawly creatures forbidden by the Torah – he would be held liable. Rabbi Shimon disagrees and frees the individual from responsibility for a shevu’ahin the latter case, since he is liable for another reason – he is mushba ve-omed me-har Sinai – he had a standing oath forbidding him from eating these things from the time of the giving of the Torah on Mount Sinai.
The Gemara on today’s daf (=page) asks why the first tanna distinguishes between food that is not edible and food that is forbidden, since from a halakhic perspective, forbidden food is also not considered edible!
Rashi explains that this question was raised for discussion, but that really the difference between the two is clear. In the first case we are discussing someone who ate non-foods, e.g., he ate dirt or drank kerosene, while in the second case what is being eaten is real food that a Jew is not allowed to eat, but is perfectly edible to others. Others suggest that these differences notwithstanding, the Gemara can be explained otherwise. When a person takes a shevu’ah, swearing that he will not eat, we must try to understand what his intention was. Logically it makes sense that he means to say that he will not eat food that is ordinarily eaten, without distinguishing between food that cannot be eaten because it is inedible and food that cannot be eaten because it is forbidden to eat.