It means to cover the refill pages ( that come in the cm albums ) completely with cardstock/paper and then add pictures, embellishments. The alternative is to scrap directly on cm refill pages.
"Wallpapering" is when you cover the whole page of your album with paper...pattern, solid or a combination. When I started to scrapbook, I was taught the CM way which was to add some accents, some journaling and your pictures to the page, but not to cover the complete page. Here are some examples of pages I have on my website that are "wallpapered".