Sheviis, Chapter 4, Mishnah 7. Regarding the Sabbatical year the Torah says that the rest of the land shall be "lachem le'ochlah" - for eating and not for waste. From here we learn that the produce is not meant to go to ruin, and that it is forbidden to destroy the produce of Sheviis, as part of the halachah that one must treat the produce of Sheviis with the sanctity of Sheviis.
For this reason it is also forbidden to squander the produce by eating it before it has ripened. This is the halachah with which our Mishnah deals: from what stage of ripening may the produce of Sheviis be used? We will also see that in certain kinds of fruit there is an earlier stage of ripening at which they may be used only in the field, even though they are not ripe enough to be brought into the house. This distinction between the field and the house is the subject of the Mishnah's discussion concerning figs.
The stages of ripening in the Mishnah:
"Me'eimasai ochlin peiros ha'ilan bashevi'is?" - From when may one eat the fruit of a tree in the Sabbatical year? And this is how the Mishnah spells it out:
"Hapagim mishe'yazrichu" - the pagim are young figs that have not fully ripened. Mishe'yazrichu, meaning from the time they have begun to redden slightly, one may eat these unripe figs together with bread in the field, since at that stage they are already fit for this purpose.
"Bichlu" - once they have ripened, one may gather them into the house. But before this stage, while they are still at the stage of "mishe'yazrichu," it is too early to bring them into the house.
"Vechein kayotzei bahen bishar shenei shavua": the stage of "bichlu," at which the fruit has begun to ripen, is also the determining point regarding tithes in the other years of the Sabbatical cycle, in which the laws of tithes apply: at that point the fruit becomes obligated in tithes, because once it has reached the stage of "bichlu" it is already considered a real fruit, and tithes must be separated from it.
In summary: our Mishnah derived from the verse "lachem le'ochlah" - for eating and not for waste - that one may not destroy the produce of Sheviis, and may not even eat it before its time. With figs there are two stages: mishe'yazrichu - one eats his bread with them in the field; bichlu - one gathers them into his house. And the stage of "bichlu," which defines the fruit as a real fruit, is also the stage that obligates it in tithes in the other years of the cycle.