A Master Project is a project which you indicate can have its layers inserted into other drawings. So, for example, if you have a common design which you need to re-use in many composite layouts, and the design is always the same size, then you may wish to make it a Master Project.
To create a new Master Project, use File - New - Master Project, or if the project already exists, change the Project Properties from the Project Browser.
To insert a layer from a Master Project into a project, use Block - Insert Master Layer.
To see which projects use a Master Project's layers, use the Project Browser.
See also Impact Browser - Advanced Operations to see the more advanced operations that can be carried out from the Project Browser, including the related topic, Linked Revisions.