When you are working with an unpublished app, you can create master measures so that they can be reused. Users of a published app will have access to the master measures, but will not be able to modify them.
A master measure is a combination of an expression and descriptive data, such as name, description and tags. You can create a master measure in different ways.
You can also create a master measure based on a measure created in a visualization. Under the measure in the properties panel of a visualization, click Add new under Master items.