Light duty was a duty status that limited an officer's workload.
In 2371, following diagnosis of a mild case of radiation poisoning, Miles O'Brien was put on light duty as recommended by Doctor Julian Bashir and ordered by Captain Benjamin Sisko, who felt the need to clarify the term for O'Brien's sake, as the chief was known for his sometimes misguided work ethic. O'Brien acquiesced. (DS9: "Visionary")
In 2372, Tuvok was put on light duty by The Doctor for three days after suffering from hyperthermic induction. The Vulcan declined, saying he did require that long to heal, and finally The Doctor agreed, with the caveat that he report to sickbay if he felt any nausea or dizziness. (VOY: "Cold Fire")