A simple, elegant, brass key. It suggests so many possibilities! What could it possibly unlock? Sadly, at the time of this writing (March 2016), there are no answers yet, as this item has not yet been implemented.
In general we know that keys are a type of consumable, and will eventually be used for unlocking things.
How to Obtain
No one knows for sure, but it wouldn't be too crazy to assume they'll probably drop as loot from chests.
How to Use
Don't you hope you can use it to unlock something really fantastic? Stay tuned!