Apparently the rabbis still argue about exactly how and when to apply the sabbath years but, from my perspective as a gentile and only an amateur student of the Bible, it's pretty simple: you grow your crops and harvest for six years and the seventh year you allow the land itself to enjoy a sabbath rest. In the close reading we've been doing in our Wednesday morning class, I came to realize that the people could walk into the field and eat the produce of the field - but they couldn't do a full-on harvest and they couldn't prune the vines, etc. — the land needed to be allowed to entirely rest but the product of the land could be consumed (the grapes didn't have to fall to the ground and become compost, at least *not all of them*). God promised His people Israel that He would provide for them in the keeping of this commandment:
‘You shall thus observe My statutes and keep My judgments, so as to carry them out, that you may live securely on the land. Then the land will yield its produce, so that you can eat your fill and live securely on it. But if you say, “What are we going to eat on the seventh year if we do not sow or gather in our crops?” then I will so order My blessing for you in the sixth year that it will bring forth the crop for three years. When you are sowing the eighth year, you can still eat old things from the crop, eating the old until the ninth year when its crop comes in. (Leviticus 25:18-22)This resonates with the dream the Lord sent to Pharaoh and Joseph interpreted, by the grace of God (Exodus 41), enabling Egypt to become a storehouse of plenty during a tremendous region-wide famine, doesn't it?