A Key is an item in Super Smash Bros. Brawl, specifically in The Subspace Emissary. True to their nature, they are used to unlock various doors encountered in the mode - if a character carrying a key touches a locked door, or the key is thrown at a door, it will be opened. Keys can also be thrown at enemies to do a decent amount of damage. If a key is broken by being dropped or thrown, it will reappear wherever it was originally found.
Keys resemble the Small Keys in Twilight Princess, although they look more futuristic.
In The Subspace Emissary, there are often locked doors. This key is the item you need to unlock these doors. You can touch a door while holding the key, or you can even throw the key at a door to unlock it. If you lose the key, it will return to the place you originally found it. This item is vital to your progress, so do your best not to lose it.