A key was a small object used to open some kinds of locks. They were often connected to key chains.
On Earth, prior to the development of voice command activation, keys were used to start vehicles such as the 1936 Ford truck that USS Voyager found floating in open space in 2371. (VOY: "The 37's")
The Akaali used keys for their doors on their homeworld. Following a fight with an undercover Malurian, Jonathan Archer took his key to Garos' curio shop. (ENT: "Civilization")
A key was used to secure handcuffs around Captain Jonathan Archer's wrists when the Ferengi were holding him prisoner. After rendering one of the Ferengi named Krem unconscious with a Vulcan nerve pinch, Sub-Commander T'Pol stole the key from him. She later teased Archer with the key, but eventually used it to unfasten the handcuffs. (ENT: "Acquisition")
In 2372 of the mirror universe, a Klingon guard accused Elim Garak of stealing the key to his collar, an action Garak patently denied. After Regent Worf stabbed him in the abdomen, the guard realized that the missing key had fallen into his boot, instead. (DS9: "Shattered Mirror")