Shevi'is, Chapter 4, Mishnah 8. In the previous Mishnah we learned the principle that one must wait before eating the fruits of Shevi'is until they have ripened properly, and we examined the measure established for this in figs. Our Mishnah deals with that very same principle, this time regarding grapes.
Two stages in the ripening of grapes:
"Haboser mishehevi mayim - ochel bo pitto basadeh" - The boser is unripened grapes. "Mishehevi mayim" - from the point that they contain liquid, enough that a little juice can be squeezed out of them. At this stage one may eat them in the field together with his bread, but it is still too early to bring them into the house, since people do not normally bring into their homes fruit that has ripened so little.
"Hiv'ish - kones l'toch beiso" - "Hiv'ish" means that the grape has ripened completely, and the sign of this is that the seeds can be seen through the skin of the grape. Once it has reached this stage, he may gather the grapes into his house.
And as was stated in the previous Mishnah as well: "V'chen kayotzei bo bish'ar shnei shavua chayav b'maasros" - in the other years of the seven year cycle too, which are not Shevi'is and in which there is an obligation of maaser, once the grapes reach the second stage of "hiv'ish" they become obligated in the separation of maasros before they may be eaten.
In summary: This Mishnah establishes two stages in grapes - "mishehevi mayim," at which casual eating is permitted in the field only, and "hiv'ish," at which the fruit is fully ripe and may be gathered into the house, and which is also the stage that creates the obligation to separate maasros during the other years of the seven year cycle.