A side sheet describes a particular type of the sheet (an interface surface) with additional content, tailored to either right or left edge of the screen. They might be of two types:
Have additional content, and stay visible while interacting with the main part of the layout. It is used for navigator and filters of desktop and tablet interfaces.
Used in mobile interfaces due to the small screen size of a gadget. Having the same type of content inside, they have to be dismissed first to interact with the main screen.