A Filler Master is a page that is inserted at the end of a Section to ensure the following Section can start on a required left or right hand page. For instance, if the next master must start on a right page, and the previous page is a right page, then a Filler Master would be inserted between them. The Filler Master can be a specifically designed master page or [None], which inserts a blank page. Since [None] is the default, it can often be left unchanged.
When configuring the Sections, the Filler Master set on the last master of a Section is the one that will be inserted before the next Section:
In the above example, every "Chapter" Section will start with a CO-Chapter Opener Master Page on the right side, followed by CC-Chapter Continued Masters on the left and right until the Section is complete. If the next Section is required to start on the right side, and the current Section just ended on a right page, then the F-Filler Notes Page Master will be inserted as a left page between the right page at the end of the current Section, and the right page at the beginning of the next Section. This is true whether the next Section is another "Chapter", or something else that is required to start on the right.