One who works in anonymity in an organization while others take on more public roles.
First used to describe the anonymous technicians and scientists who worked behind the scenes in the UK during World War Two. Lord Beaverbrook coined it in a speech in March 1941:
"Now who is responsible for this work of development on which so much depends? To whom must the praise be given? To the boys in the back rooms. They do not sit in the limelight. But they are the men who do all the work. Many of them are Civil Servants."
The expression may date from earlier than 1941 as, by April 1942, it was the title of a comic film by the popular English variety artist Arthur Askey.
Note that a related word - boffin, also emerged around the same time, seen in this example from just at the end of WWII:
The (London) Times, September 1945 - "A band of scientific men who performed their wartime wonders at Malvern and apparently called themselves 'the boffins'."