Maaser Sheni, Chapter 3, Mishnah 5. This mishnah deals with the law that once a person brings fruits of Maaser Sheni into Jerusalem, he can no longer take them out again; they must remain in Jerusalem and be eaten there.
The words of the mishnah:
"Ma'os nichnasos li'Yerushalayim v'yotzos" - money of Maaser Sheni may be brought into Jerusalem and taken back out, and there is no concern about this at all.
"U'peiros nichnasin v'einan yotzin" - once fruits of Maaser Sheni have entered Jerusalem, they cannot leave. The expression used for this is that the partitions of Jerusalem, its boundaries or its walls, absorb them - they take hold of them, and there is no longer any way to take them out. With money of Maaser Sheni, however, there is no such rule.
The opinion of Rabban Shimon ben Gamliel:
"Rabban Shimon ben Gamliel omer: af ha'peiros nichnasin v'yotzin" - he disagrees with this rule regarding fruits, and holds that fruits too, after they have entered Jerusalem, may be taken back out.
The Yerushalmi explains, however, that even Rabban Shimon ben Gamliel, who says that the fruits may be taken out, agrees that the partitions of Jerusalem take hold of the fruits to a certain degree: they may not be taken out of Jerusalem in order to be redeemed outside. The permission is only to take them out temporarily - for example, to grind the wheat of Maaser Sheni and bake it outside Jerusalem, and afterward to bring it back into Jerusalem and eat it there. Why would a person want to do this? Outside Jerusalem, mills and ovens were more readily available, and it was cheaper. That, then, is the reason it was permitted to do so on a temporary basis.
This alone is what Rabban Shimon ben Gamliel means when he says "peiros nichnasin v'yotzin" - only on this temporary basis. He does not mean that they may be left outside Jerusalem permanently: eating them there is certainly impossible, and redeeming them there is impossible as well.
In summary: in this mishnah we learned the distinction between money of Maaser Sheni, which enters Jerusalem and leaves it without any concern, and fruits of Maaser Sheni, which the partitions of Jerusalem absorb so that they do not leave. According to Rabban Shimon ben Gamliel, the fruits too enter and leave, and according to the explanation of the Yerushalmi this is only a temporary departure for the purpose of grinding and baking outside, followed by a return to be eaten in Jerusalem, but not for eating or redeeming outside Jerusalem.