Mitzvah #84 was a commandment to render produce ownerless during the Sabbatical year. Here, we are commanded to let the land lie fallow and unworked for that year.
The actual verse may be unclear. In its entirety it reads, “Six days you wall work and on the seventh day you shall rest; you shall refrain from plowing and harvesting.” From a surface reading, it would seem to be forbidding agricultural labor on Shabbos (which also happens to be true), but we see from Makkos 8b that the latter half of the verse is actually talking about Shemittah. We see this more overtly in parshas Behar (Leviticus 25).
Even though it seems to be telling us not to work the land, the command to rest or to refrain from doing something is a positive mitzvah. This is the case for Shabbos, Yom Tov and Shemittah. (See Talmud Shabbos 24b.)
As we discussed in Mitzvah #84, the reason for this mitzvah is similar to the reason for Shabbos. We refrain from acts of creative labor on Shabbos in recognition that God is the Creator; we refrain from working the land in the Shemittah year in recognition of the fact that the land is His.
This mitzvah applies to both men and women, but only in Israel. Some authorities say that it is Biblically binding in this day and age, while others are of the opinion that the obligation is currently rabbinic. This mitzvah is the topic of the tractate of Mishna called Sheviis and, as we have mentioned, it is discussed in Makkos on page 8b. It is codified in the Mishneh Torah in Hilchos Shemittah and it is #135 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos. It is #21 of the 26 mitzvos that can only be performed in Israel according to the Steipler Gaon.