Rest was a period of inactivity or relaxation in order to recover energy.
In 2268, Captain James T. Kirk told a group of orphaned children that they could use some rest after having a busy day. (TOS: "And the Children Shall Lead")
According to Doctor Beverly Crusher in 2364, cell division in single-celled lifeforms, at least organic ones, was preceded by a resting state. (TNG: "Home Soil")
In 2374, after the Prophets granted Benjamin Sisko a miracle, they warned him that there was a penance associated with it: that he was of Bajor, but he would find no rest there. (DS9: "Sacrifice of Angels")