Rav Dimi quotes Rabbi Yoḥanan as teaching that a shevuat sheker is a false oath taken regarding the future that is not kept, while a shevuat shav is an oath in vain about something that happened in the past. The Gemara on today’s daf challenges this opinion with a baraita that teaches that these two are the same, but explains that this means that the two were taught simultaneously – they were said at the same time in a manner that allowed the listener to realize that two similar laws were being taught and understand the nuance of difference between them.
A parallel teaching that uses this same concept is the divergent readings in the Ten Commandments, where we find that there are differences between the “first tablets” in Sefer Shemot (chapter 20) and the “second tablets” in Sefer (chapter 5). One of those differences relates to Shabbat, which we are commanded to “observe” (shamor) and to “remember” (zakhor). The explanation is that these two concepts were said simultaneously (shamor ve-zakhor be-dibur eḥad), from where we derive laws like the fact that women are obligated in Kiddush on Shabbat.
The challenge regarding women and stems from the fact that Shabbat appears to be a mitzvat aseh she-hazeman gerama – a positive commandment dependent on time – a type of commandment from which women are usually exempt. The Gemara teaches that since shamor – which refers to the negative commandments of Shabbat – and zakhor – which refers to the positive commandments, like Kiddush – are viewed as connected since they were uttered simultaneously, they are seen as coming into effect together. For this reason, women, who are obligated in refraining from the negative commandments of Shabbat, are required to perform the positive commandments, as well.