The Codes window allows you to unlock tricks or redeem codes found in the game on it.
You may find it if you click the arrows on the inventory interface.
Codes may be found from Tombstones, and they are redeemable only once.
But secrets can be used all time you want, but there is no way of discovering
what are the secret codes right now.
The codes found in the game are Map side, meaning only players playing on that map will be able to use it.