This Mishnah discusses a joint venture between a Jew and a non-Jew. For example, the Jew approaches the non-Jew and says, “You have the facilities, you have the wine press, you have everything needed to produce the wine, but you don't have the ability to make it kosher. Let me make the wine in your place. I'll make it kosher, and we'll be able to market it to Jews as kosher wine.” So here we'll discuss whether he is allowed to leave that completed wine on the idol worshiper's property without it becoming forbidden to benefit from.
The Mishnah says that someone is preparing an idol worshiper's wine to be kosher. They understand that he'll pay the idol worshiper for the wine after he sells it. Until he's selling it, or until it's ready to be sold, he wants to store the wine on the idol worshiper's property, in a house in the reshut harabim (public domain), facing the public domain, out there in public. If it's in a town where there are both Jews and Gentiles, the wine would be permitted to drink. The idol worshiper is in this spot. It's open to the public, and there are Jews around. In a way, everyone serves as the quasi-mashgiach (watchman) for this type of wine. The non-Jew would be afraid to enter the building in case the Jew saw him, because he knows that all the wine would become unkosher. Therefore, that would be considered enough for the wine to remain kosher and permissible to benefit from, even though it's within the non-Jew's property.
But if it was in a town where all the townspeople were idol worshippers, then the wine would be prohibited to benefit from unless there was an appointed mashgiach, in a way, a Jewish watchman, who would watch over the wine and make sure nobody came in. In that case, the non-Jew is not going to enter. If you don't have a watchman, then the idol worshiper is not going to be afraid to enter, because no Jews would see him and report him as being not kosher. The watchman, the mashgiach we're talking about, doesn't have to sit there and guard the wine the entire day. Even if he goes out and comes back from time to time, the wine is considered permitted as long as he's around occasionally.
What we just spoke about was wine left on the property of the idol worshiper whose wine was prepared to be kosher. The implication is that if he stored it on the property of another idol worshipper, it would be permitted even if there were no Jews living in that town. The person who is not the owner of the wine would be nervous to touch it or be around it, lest people think he's entering a spot where he doesn't have permission to be, or that he's a thief. But this next part of the Mishnah disagrees: “No, it doesn't matter if he's the owner or not. All idol worshippers are the same.” In other words, the wine is prohibited no matter what when you're in a town where all the people are idol worshippers. It doesn't matter whether it was stored on the property of the man whose wine was prepared to be kosher or on the property of another idol worshipper. It simply doesn't matter if it's within the realm, if it's accessible to an idol worshipper. You have to be nervous that they're going to use it as a libation on the Mizbeach (altar) for Avodah Zarah (idolatry), and therefore it becomes problematic no matter what if there's no Jew overseeing the process.