When kosher (fit under Jewish law) wine is left within the vicinity of an idol worshiper, a Jew has to guard it to make sure that the idol worshiper is not going to touch it and make it forbidden because of... But you don't have to have a Jew there constantly watching at all times. A Jew could simply leave the idol worshiper alone with the wine, with the fear that he'll come back and a Jew is going to catch him tampering with it.
We'll continue to talk about that over the next few Mishnayot. Mishnah says: If there was an idol worshiper who was helping a Jew move jugs of wine, barrels of wine, from one place to the other, and the Jew left the Gentile momentarily alone with the wine, the status of the wine depends on what happened. If the wine was left in a state of supervision where the idol worshiper thought that the Jew could return at any moment—that he's about to pop back in—and he had no clue where the Jew went, then the wine is permitted because the idol worshiper is afraid that the Jew is going to return at any moment and catch him touching the wine. If the Jew told him that he was going to be far, then the wine would be prohibited.
If the Jew stayed away long enough for the idol worshiper, number one, to make a hole in the seal of the barrel, seal the hole, and then allow it to dry before the Jew returned, then, since the idol worshiper knows that the Jew is going to be away for a little bit of time, he's not going to be nervous to tamper with it, to play with the barrel, or to do something to the wine, because he's confident that he'll have the ability to hide all of the evidence before the Jew comes back. So therefore, the wine would be prohibited if the Jew stayed away long enough for all of this procedure to get done. But the next Tanna (Mishnaic sage) says that the wine is not prohibited unless the Jew is away for an even longer amount of time. The wine would only be prohibited if the Jew stayed away long enough for the idol worshiper to open the barrel, remove the seal, make a new seal, and then allow it to dry. That would be a longer amount of time. The Tanna says that the idol worshiper wouldn't risk doing what he's doing—opening up the barrel—unless there was an extended period when he could really cover up the evidence and make sure that there's no lingering effect of him opening up the barrel.