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")