Usually a key or set of keys that are used to control elevators in buildings in the event of a fire. Fire deparments usually have a set of keys that control most elevators.
Posession of such keys can be handy if you regularly use elevators in buildings you shouldn't be in. However these keys can be considered to be "like" lockpicks and as such it can be illegal to have said keys in your possession unless you have a lawful use for them in certain countries and states.
Can also mean a "Break glass in event of emergency" key that is used to unlock a door that is usually locked or to turn off a halon fire system