This Mishnah talks about someone who swears not to eat or drink and then eats or drinks something that is either inedible or already prohibited by the Torah. The Mishnah says that someone swears he is not going to eat and then eats foods that are not fit to eat. For example, he might eat a scoop of dirt or drink something unfit to drink. In that case, he would not be obligated to bring a korban (offering), because eating or drinking something that is not food is not considered eating or drinking.
If a person, in the second scenario, swore that he wasn’t going to eat and then ate an animal that died without Tereifot, the discussion also includes Tereifot: Tereifot is an animal that was sick and was going to die within 12 months. Shekatzim are types of bugs, worms, snakes, scorpions, rodents, and other insects. U'remasim are types of tiny worms that grow inside fruit or rotting meat. If the person ate any of these things, Chayav—he’s obligated to bring a korban according to Tanna Kamma (first tanna). These foods are edible, and even though the Torah does not allow us to eat them, they are still considered foods, and therefore he made a shevuah (oath) against them. Rebbe Shimon poter; he exempts this person from having to bring a korban for that reason. The Torah already forbade them, and he says there is an idea called “a prohibition cannot take effect on a pre-existing prohibition.” The person is already not allowed to eat these foods according to the Torah, so the oath he made not to eat them does not affect them whatsoever, and therefore the person is patur (exempt). They are already forbidden, so the oath did not catch onto them. Hachamim (sages) look at it differently. Tanna Kamma says that even though the rule is ein issur chal al issur (a prohibition cannot take effect on a pre-existing prohibition), the shevuah was not to eat, and therefore, as part of the shevuah, the act of eating is the problem, whether the food he is eating is problematic or not. Therefore, it would take effect on permitted foods and on the foods he eats, including forbidden foods as well. That’s the opinion of Tanna Kamma.
In the third scenario of the Mishnah, if a man says konam—if you remember Masechet Nedarim, konam is a term used to create a neder (vow)—a person, on a Torah level, can prohibit other people from benefiting from his own things by making a neder. Here, a man makes a neder. He says, “If I eat today, my wife will be forbidden to benefit from me.” Then what happens? He eats all these forbidden foods, which are already forbidden by the Torah, and his wife becomes prohibited from benefiting from him. These foods are edible; they are food, and therefore eating them is considered a fulfillment of that condition. The condition was, “If I eat,” and he ate, regardless of whether the foods are forbidden or not. Therefore, the neder takes effect, and his wife is not allowed to benefit from him. We would say this works even according to Rebbe Shimon. Rebbe Shimon, who held to that rule, ein issur chal al issur, would say in this case that it is not about whether there is a prohibition; it is about the act of eating—“if I ate.” He would agree here. So the person here made the konam: “I make a neder: If I eat, my wife is going to be prohibited from benefiting from me.” He ate, so therefore his wife is prohibited from benefiting from him. So even Rebbe Shimon would agree.