If a person has forbidden food cooked with permitted food, and the permitted food absorbs the taste and flavor of the forbidden food, it becomes forbidden as well. This is known as ta'am ke'ikar. The flavor of the food is considered to be like the food itself. Since the flavor of the forbidden food has been absorbed and become part of the permitted food, a person is not allowed to eat those foods, because he would be eating the flavor of forbidden food. This is a machloket (dispute): is this de'oraita (Torah-level) or derabbanan (rabbinic)?
This even applies when the foods have the same taste. Let's say you have two pieces of meat together. One was not kosher and one was kosher, and the flavor of the forbidden one entered the permitted one. Even though technically they have the same flavor, it still becomes problematic. The flavor of the forbidden food is in the permitted food, and therefore it becomes a halachic problem. This does not apply when there is so much permitted food—such a large amount—that the flavor of the forbidden food becomes very diluted in the pot. In this type of situation, we say there is bitul. The forbidden food becomes nullified, and the permitted food is allowed to be eaten.
We'll talk about the halacha (Jewish law) if the thigh was cooked with the Gid HaNashe (sciatic nerve) still in it. The Mishnah says: If a thigh was cooked with the Gid HaNashe still in it, and the thigh is of a size such that the Gid HaNashe could possibly give forth flavor, the entire thigh is forbidden to eat. This is because the flavor of the Gid HaNashe is absorbed into the thigh during the cooking process. How do we measure it? In other words, if the taste of the Gid HaNashe is exactly the same as the taste of the meat, how do we figure out whether there is a transfer of ta'am (taste) here? We can't taste it. No. What do we do to understand whether there is a transfer? How do we measure this?
The Mishnah says that we should imagine meat giving flavor to turnips. If you had a piece of meat the same size as the Gid HaNashe and put it together with turnips that had the same volume as the thigh, the mashal (analogy) is clear here: meat to turnips, and the Gid HaNashe to the thigh. So let's just replace the Gid HaNashe-to-thigh comparison with meat-to-turnips. If the meat, which had the same volume as the Gid HaNashe, were put with turnips that had the same volume as the thigh and cooked together, would the flavor of the meat be transferred to the turnips or not? If the answer was yes, then in our situation the problem is considered to be a transfer of ta'am. But if the thigh were larger, so that the flavor of the meat the size of the Gid HaNashe would not actually give any taste to the turnips because they were so large, then there would be no transfer of ta'am. The Gid HaNashe would become nullified in the meat of the thigh and would be permitted. We'll elaborate more in the next Mishnah.
According to many, this is where we learn the 1-to-60 measure. What we said about the meat and turnips being equal to the Gid HaNashe and the thigh is referring to a very large thigh in relation to the Gid HaNashe. If they were cooked together, there would be no transfer of ta'am if there were at least one-sixtieth—1 to 60: one part asur (forbidden), 60 parts mutar (permitted). It would be permitted. If there were less than 60 parts, for example, 1 to 59, we assume that the 59 is not enough for the forbidden food to become nullified. It actually picks up ta'am and would become problematic. So we learn from this Mishnah, from this situation, the 1-to-60 ratio that would allow it.