A duvet cover is a protective sleeve that covers your duvet, much like a pillowcase on your pillow. In addition to protecting the duvet, it allows you to easily change the look of your bedding, without having to replace the entire duvet. Duvet covers are easily cleaned, and compact for storage.
Some skip the top sheet when making a bed with a duvet, because the duvet cover acts much the same way as a sheet. This is common practice in Europe, where duvets are heavily favored. If you’ve heard someone say they sleep European, this is what they mean!