The Folder Filters Page contains the settings for controlling how DiffMerge filters out uninteresting files and sub-folders from the Folder Diff Window display.
The Restore Defaults button restores all the fields on this (and only this) page to their original factory settings.
Scanning the file system is very expensive and time consuming. The use of Folder Filters can usually speed this up considerably by eliminating the need to look at various types of files and entire sub-folders. It will also help reduce on-screen clutter.