As you are journeying through Edrea to find out more about the Luminary and seeking ways to defeat the Dark One in Dragon Quest XI, you will come across a mysterious red door in your travels that are locked. Behind these red doors is treasure that will be worth your time but unfortunately, it wont be accessible until you are midway through the game.
Source: N4G PC Dragon Quest XI Guide – The Magic Key and the Locked Red Door Locations