Page thumbnails are what the reader sees when pausing their motion book.
In the image above you can see the page thumbnails at the bottom.
Making thumbnails is very simple:
1) Open your effects rollout by clicking the arrow next to the effects scrubber bar (highlighted in red below).
2) Place the effects scrubber (highlighted in blue below) where ever you want to on the timeline to get the image you want, then click on the camera icon at the left (highlighted in red below).
...And that's all there is to it! You've now set a page thumbnail for that page, so when your readers pause or scroll through the book they can get an idea of what each page is about.