Thumbnails and Side to Side navigation are new Page Movement options in Word 365 (2019) for Windows. Here’s how they work.
On the View ribbon, there is now a Page Movement area where you can select from the familiar Vertical scrolling view or the new Side to Side option. Microsoft describes the side-to-side view as flipping pages, like a book. This basically gives you a two-page view. But while there is a bit of a flip transition, it’s not flipping folios, it’s more like having a printed manuscript laying on your desk, and sliding pages to the left as you read.
So, paging down (or pulling the slider) doesn’t take you from viewing pages 2–3 to viewing pages 4–5, it goes from 2–3 to 3–4 to 4–5, etc. This must be useful for something, but I find the Multiple Pages view with vertical scrolling better because it maintains folios or, moreover, because it doesn’t show me the same page twice (once on the right, then once on the left). You can see that option in the screenshot of the ribbon, above. Note that it is greyed out when Side to Side is selected. The Multiple Pages view is only available in Vertical Page Movement mode.
Note that Side to Side is very different from the side-by-side view, which arranges two separate documents, in separate windows, side-by-side on the screen.
The Thumbnails option on the View ribbon shown above only becomes visible after selecting the new Side to Side option in the Page Navigation area. (Again, only on Windows.) This fits about two rows of pages across your screen to use for navigation only. Clicking on a page takes you that page. You can’t interact with the pages (do editing) in Thumbnail view.
Again, I prefer the Multiple Pages view (zoomed out to 25% or so) because you can still edit the words in that view, rather than just navigating the document. For navigation using thumbnails, I prefer the Pages option in the Navigation Pane that’s always been there.
Options on Mac
Sorry, Mac users, you don’t get these options (yet?). Not that it’s much of a loss. You can create these effects, with more usability, using Multiple Pages view and the Navigation pane as described above.