The master node acts like a template that will be used across all nodes within a tour.
For instance, you can add user data, a patch, or hotspots and these would appear on all nodes. You can edit the Master Node by going to the Tour menu and choosing, Edit Master Node.
You can use placeholders in the Project User Data of the Master Node which can pre-populate the fields. For example, if your image filenames include the title of the image, you can use the placeholder,
$(ib) in the Title field.
Then, when you exit the Master Node you will see the User Data title populated.