The Flip Movies consisted of little booklets that contained photos stills from various Universal monster movies. When the edges of the pages were riffled, the illusion of the photos being flipped by quickly provided a very brief animation for kids to enjoy. (Remember, this was decades before kids could carry little video-text-phone devices in their pockets; portable animation just wasn't heard of until recently.)
The booklets each contained 32 pages, and came attached two at a time, sold in 5-cent packs.
Note that these are only considered to be in higher condition if both booklets are still attached to one another.