"But the seventh year you shall let it (the land) rest and lie fallow, that the poor of your people may eat, and what they leave will be eaten by the beast of the field." (Shmot 23:11)
"In plowing time and in harvest time you shall rest." (ibid. 34:21)
"It shall be a year of solemn rest for the land." (Vayikra 25:5)
Abandoning ownership of all agricultural produce of the seventh (shemita) year so that anyone who wishes to acquire this produce can do so.
To cease working the land in the seventh year.
To remind us every seventh year that G-d created the world ex nihilo in six days and rested on the seventh.
In addition to resting from agricultural labor in the seventh year as we rest from all labor every seventh day, we are also commanded to leave ownerless all that the land will produce in this year to remind us that the land which grows produce for us every year does not do so by its own power but that there is a L-rd over it who truly owns it and its earthly owner.