Our Gemara introduces a teaching about the behavior of tzenu’im – modest, or, in the context of our Gemara, uniquely God-fearing individuals – who, when they had a vineyard of revai would set aside money, saying, “Anything that is harvested (by others) from this vineyard will be redeemed on this money.”
The Torah commands (see Vayikra 19:23-24) that the first three years after trees or vines are planted, the produce is orla – any fruit that is grows is forbidden. The produce of the fourth year will be neta (or kerem) revai – it has a unique status of holiness, similar to that of ma’aser sheni, requiring the fruit to be taken to Jerusalem and eaten within the city walls. In the event that the fruit cannot be taken, it can be redeemed and the value of the fruit (plus one-fifth) must be taken to Jerusalem where it is used to purchase food.
The Mishna in Massekhet Ma’aser Sheni teaches that the owner of a field whose trees are orla or whose fruit is neta revai must make various signs on his property so that the unsuspecting passer-by will not eat any of the fruit. The Mishna concludes that the tzenu’im are even more careful and arrange for neta revai fruit to be redeemed, so that anyone who takes from the field will not fall victim to eating forbidden fruits.
According to the Rambam, the behavior of the tzenu’im was done only during the Sabbatical year, when the fruit may have been taken by anyone. Others suggest that this was done by tzenu’im in other years, as well, out of concern even for those who took fruit without permission.