An efficiency rating was a measurement of how efficient an individual or group was.
In 2267, Spock and Pavel Chekov created a diversion at Captain James T. Kirk's request by manufacturing a loud argument. The Vulcan "complained" about the efficiency of Chekov's tricorder readings; Chekov told him to mind his own business, saying he had a very high efficiency rating. (TOS: "The Apple")
In 2285, Kirk surmised correctly that Spock was remaining behind near the bridge simulator after the completion of the Kobayashi Maru scenario to find out what efficiency rating he planned to give Spock's cadets. (Star Trek II: The Wrath of Khan)
In 2376, Seven of Nine gave USS Voyager's Operations division an efficiency rating of 76 percent. She explained further that the night shift had little work to do once the course had been laid in. (VOY: "Good Shepherd")