Challah, chapter 2, mishnah 1. This mishnah deals with a fundamental question: is the obligation of challah unique to the Land of Israel alone, or does it apply as well to grain that originated outside the Land?
"Peiros chutz la'aretz shenichnesu la'aretz":
The mishnah opens with "peiros chutz la'aretz" - produce from outside the Land. The word 'peiros' here refers to grain, and in our case specifically to one of the five species of grain that are relevant to the mitzvah of challah. This grain grew outside the Land of Israel, but it was brought into the Land and made into dough there. The mishnah rules that it is obligated in challah, even though the grain itself grew outside the Land.
This law is derived from the verse: "el ha'aretz asher ani meivi eschem shamah" (to the land to which I am bringing you) - from the moment you enter the Land to which I am bringing you, the obligation takes effect upon you, whether with produce from outside the Land or with produce of the Land, so long as the dough is made in the Land of Israel.
"Yatza mikan l'sham" - the dispute between Rabbi Eliezer and Rabbi Akiva:
The mishnah continues with the reverse case: a person who leaves from here, from the Land of Israel, to there, to outside the Land, taking with him grain that grew in the Land of Israel, but who will make the dough only once he is outside the Land. On this the Tannaim disagree:
Rabbi Eliezer obligates - in his view the grain is obligated in challah, since it comes from the produce of the Land of Israel.
Rabbi Akiva exempts - in his view it is the kneading of the dough, that is, the turning of the grain into dough, that must take place in the Land of Israel. Since the kneading was not done in the Land, the origin of the grain does not suffice to obligate it.
In summary: in this mishnah we learned two sides of the same question. Produce that grew outside the Land and was brought into the Land and made into dough there is obligated in challah, as derived from the verse "el ha'aretz asher ani meivi eschem shamah." But regarding produce that grew in the Land and was taken from here to there, the Tannaim disagree: Rabbi Eliezer obligates because of the origin of the grain, while Rabbi Akiva exempts, since in his view the kneading of the dough is what determines the matter, and it must be done in the Land of Israel.