Master Mode is a more challenging mode of the game that can optionally be played after purchasing the Breath of the Wild Expansion Pass.
How to Unlock Master Mode
To use Master Mode, you must purchase the Breath of the Wild Expansion Pass. You can do so by choosing Downloadable Content on the game's Title Screen. Afterward, when you return to the Title Screen, there will be a "Play in Master Mode" option.
Overview of Master Mode
Master Mode is more difficult than Normal Mode, and it only allows you to have one save file and one autosave file. These save files are separate from your Normal Mode save files, so you will not lose your previously saved Normal Mode game if you start a game in Master Mode.
Master Mode Differences
The loading screen for Master Mode is red instead of white. There is also a Triforce in the corner of the screen if you are playing in Master Mode.
In Master Mode, enemies are stronger. All enemies will regenerate health after taking damage. Enemies that have multiple levels (such as Bokoblins, Blue Bokoblins, Black Bokoblins, etc.) will be one level higher than in Normal Mode, and there is an additional Gold level of enemy. There are specific locations where the lowest level enemies can be found, if needed, for the Hyrule Compendium.
There are also floating enemy strongholds in certain areas, similar to enemy strongholds on the ground.