When you create and build your visualizations, you can save assets to reuse in other visualizations and on other sheets. You can save visualizations, dimensions and measures, as master items in the assets panel. When your app gets published these master items will be available to others as ready-to-use visualizations, dimensions and measures.
Any updates you make to the master item are applied everywhere the master item is used. For example, you could use a master measure in as many of your visualizations as you like while only having to update it in a single instance to update all instances of the measure in your visualizations.
One of the purposes with creating and maintaining master items is for other users to explore their own ways and directions in the data, on top of what you have provided in the app as pre-made sheets with visualizations. The users will be able to create their own visualizations with your pre-made master dimensions and master measures, for example.