It is a storage device that can only hold Mystcraft Pages (Symbols, Link Panels, and/or blanks), but can hold an unlimited quantity. It keeps them in order without sorting or stacking, as compared to the Symbol Portfolio.
The GUI allows pages to be dragged around, and pieces of paper can be placed into the folder to make blank pages. When dragging pages into the folder, either left or right click can be used to drop them in, but only on an empty space in the black area. If the visible area is completely covered with pages, it will be necessary to scroll down to expose (at least part of) a blank area below the pages, where new pages can be dropped.
Folders are useful for gathering and sequencing the symbols for an intended Descriptive Book; they can be set up in a Writing Desk, then taken over to the Book Binder and "dumped in" by clicking on the gray page list area with the folder on the cursor.