Creative Mode is a game mode added during the 1.4 update that allows you to have unlimited resources and you decide to have enemies or not during gameplay. Originally, the gamemode was called "Infinite Resource mode", until changed in recent updates. Mostly, it behaves like Survival and Free Build put together, but you can only unlock this game mode by redeeming a code from CastleMiner, the original Indie game this was based upon.
The player has to obtain a block first, before "duplicating", or infinitely placing it. When crafting an item, you can produce an infinite amount of the item, until your inventory is full. Bullets in this gamemode are not affected either, so you also have unlimited ammunition.
Note: "Promo codes vary between consoles. Each console includes a promo code from CastleMiner that only works on the same console when entered in CastleMinerZ."