Food could only become tamei (impure) if it is prepared properly to receive the tumah (impurity). In other words, two things have to happen. First, the food has to come into contact with a liquid. The abbreviation is Yad Shachat Dam, which, in English, is dew, water, wine, olive oil, blood, milk, and honey. Likewise, the owner of the food has to want it to come into contact with one of those liquids. At that point, even after it dries off, it is prepared for tumah. If something tamei touches it, it becomes tamei.
When talking about olives, they're usually pickled in a certain type of brine, a salt. When they're in that salt, liquid comes out of the olives and wets the surface. The liquid, which we know as olive oil, comes out of them. Therefore, the olive itself is prepared and capable of becoming tamei. But the question is, do we assume that the owner wanted that contact or not? If he desired that contact, then the food would become tamei. The olives would become tamei. But if he did not, then even though the liquid was there, the food would not become tamei.
The Mishnah says: If a person has a barrel of olives that are pickling in salt, he does not have to make a hole in the barrel to show that he wants the liquid to come out. We assume that he does not want the liquid to be on the olives. We can make that assumption. He doesn't need to do anything to reveal that to us. If he does nothing, according to Beit Shammai, even here, the liquid would not prepare the olives for tumah. Or Beit Hillel, he does have to make a hole in the barrel. In order to show that he does not want the liquid to be there, he has to make a hole in the barrel. If he doesn't make a hole, then we assume that he's okay with the liquids being there, and therefore the olives are now prepared for tumah.
But Beit Hillel would agree in one case. Beit Hillel would say that if he did make a hole in the barrel, but then sediment from the olives blocked the liquid from draining out, the olives cannot become tamei. The owner made a hole. By making a hole, he revealed what he really wanted: he did not want the liquid. So if the liquid got trapped in there by sediment, that was not the owner's desire. Therefore, according to him, the olives would not be prepared for tumah, even if the hole was blocked later on.
The Mishnah continues with a machloket (dispute) between Beit Shammai and Beit Hillel about tamei oil and how we could purify it. Usually, a mikveh (ritual bath) only works to purify people and utensils. But when it comes to liquids or foods, a mikveh doesn't work. The question is: If there was tamei oil on somebody's body and then he dipped into the mikveh, is that oil looked at as part of the person's body and therefore pure, or is it separate from the body, so the mikveh wouldn't work for it?
Somebody put pure oil on himself, and then he became tamei and transmitted the tumah onto the oil as well. But then he went down to the mikveh and dipped into it to purify himself from the tumah. Even if he's dripping oil afterward, that means the oil was not absorbed into his skin and is not part of his body. The oil is still tahor (pure). We treat the oil as part of his body, and therefore, through his dipping in the mikveh, the oil became purified. The point of saying it's dripping is to show that it doesn't matter how much oil is there. No matter what, the oil that's on his body becomes purified. Only if a small amount of oil is on him at that time, enough to spread over a small limb. At that point, a tiny amount of oil in relation to his body is considered to be part of his body, and dipping in the mikveh would work for that as well. But if a large amount of oil is there, it's considered to be its own entity, not part of his body, and therefore the person would stay tamei even though he dipped in a mikveh.
In this previous machloket, the person put the pure oil onto his body and afterward became tamei and transferred the tumah onto the oil as well. In the next case, the oil was tamei before he put it onto himself. Then we have a machloket between Beit Shammai and Beit Hillel about the maximum amount of oil he could put on himself in order to make himself tahor. Beit Shammai goes down to that lower amount after he dipped in the mikveh. If only enough oil is on him that he could smear it onto a small limb of his body, then we treat the oil as part of his body, and the oil is purified with his dip in the mikveh as well. But if there's a large amount of oil, even Beit Shammai would say here that the oil would remain tamei.
If the oil on his body has the ability to moisten anything when it's touched, then the immersion makes it tahor. But anything more than that—if a larger amount of oil is on his body, which would be like an eiver katan (small limb), the amount of an eiver katan, which is Beit Shammai's opinion—then the oil would remain tamei according to Beit Hillel. So here, when the oil was tamei and then he put it on his body, the amounts would be lower in order to purify it by a mikveh, according to both Beit Shammai and Beit Hillel. But according to a different version of Beit Hillel's teaching, the maximum amount of oil that he could put on that would become tahor is greater.
Rebbe Yehuda says in the name of Beit Hillel that if the body is so moist that the oil he touches on his body is not only moist but could also moisten something, then the oil is tahor. So if the amount of oil is considered to be lower than that, it's purified by the immersion. But if it's larger than that, if it's more than that, then it would remain tamei. So according to this opinion of Beit Hillel, it's not only tofeach, it's not only moist, but it's moist and has the ability to moisten something else. That's the maximum amount. If the oil was already impure, and he put it onto his body and then dipped into a mikveh in order to purify the oil...