While discussing the four domains (public, private, exempt and the Rabbinic karmelit, see yesterday’s daf, the Gemara quotes a baraita that teaches
If he carried out an object on from the private domain to the public domain or vice versa, if he carried in, if he did so unwittingly, he is liable to bring a sin-offering. If he did so intentionally and there were no witnesses to his act and he was not forewarned, he is punishable from the hand of Heaven with the punishment of karet. If he was forewarned and there were witnesses to his transgression, he is punished by the court and stoned.
In response to this teaching, the Gemara asks why there is a need to emphasize the punishments that are meted out. Surely we are aware that someone who performed a prohibited act on will be punished with karet or stoning! The Gemara answers: This came to teach us in accordance with the statement of Rav, as Rav said: I found a hidden scroll in the house of Rabbi Ḥiyya in which matters of Oral Torah were briefly summarized, and in it was written: Isi ben Yehuda says: The primary categories of prohibited labor on are forty-less-one, and he is liable only for one.
This obscure statement is explained by the Gemara to mean that there is one of the 39 prohibited activities on that is not punishable by stoning; it remains a simple negative commandment. The baraita is emphasizing that the prohibition of transferring an object from one domain to another is not that activity – it is punishable by stoning.
Regarding the “hidden scroll” that was found by Rav: For many generations, it was prohibited to write the contents of the Oral Torah. Due to the exigencies of the time, it was decided to redact the Mishna and write it down. Nevertheless, even when it was prohibited, Sages would summarize important matters in brief notes to help them remember. These scrolls were not published and were, therefore, referred to as hidden scrolls. According to the ge’onim, these scrolls were known as hidden because they were anthologies of halakhot that were not universally known, even though they were not concealed intentionally.