According to the Torah, until the omer offering was brought on the second day of Pesach, the new grain harvest could not be consumed (see Vayikra 23:14). The Mishnah on today’s daf (=page) records that once the korban ha-omer was brought, one could enter the Jerusalem marketplace and immediately find that flour from the new harvest was available for purchase. Rabbi Me’ir says that this situation existed against the wishes of the Sages; Rabbi Yehudah says that it was done with the permission of the Sages.
The rishonim disagree about what the Sages may have found to be objectionable. Rabbenu Gershom explains that the Sages were concerned with the fact that the harvest took place before the omer was harvested and brought to the Temple, lest people come to partake of the new grain before the proper time. In his Commentary to the Mishnah, the Rambam explains that there was nothing untoward with the harvest, the concern was with the ready availability in the marketplace even before the offering had been brought.
The Gemara asks why Rabbi Yehudah does not appear to be concerned lest people eat from the new crop while it is still forbidden, since we find that on the day before Passover he permits searching for hametz only when it can still be eaten; once it is forbidden to eat he becomes concerned lest a person eat it accidentally should he find it.
While both Abayye and Rava offer suggestions in the Gemara explaining why Rabbi Yehudah may be inclined to show concern about eating hametz after the time that it becomes forbidden, even though he is not worried about eating new grain before it becomes permitted, in his Netivot ha-Kodesh Rabbi Avraham Moshe Salman offers a simple distinction. When a forbidden food is a simple lav – a negative commandment – Rabbi Yehudah is not concerned lest it be eaten accidentally. Hametz on Pesach, however, which carries with it the severe punishment of karet, demands a more restrictive Rabbinic prohibition.