A gaffer is a member of a movie or television crew. The gaffer runs the production's electrical department.
One of the jobs credited at the end of a film is the gaffer. A gaffer is responsible for the electricity that powers the lights, so her job is vital to the success of the movie. You can also call a gaffer a Chief Lighting Technician, and you can call her assistant a Best Boy. The word gaffer is also sometimes used to mean "old man," which is the oldest meaning of the word, originally a contraction of godfather.