The Weapon store key is an item used during the Shield of Arrav quest to access the Phoenix Gang's weapon storage room. In order to obtain it, you must either be in or have a partner in the Phoenix Gang who can get the key.
This item was previously named just "Key" until the Varrock update, when it was renamed to "Weapon store key", possibly to avoid confusion with other keys.
If you lose the key, you can get another one by talking to Straven.
This key can be stored on the steel key ring.