In the December 2018 SharePoint roadmap "pitstop" Microsoft announced many new enhancements for SharePoint Online. One of these is the use of so-called site designs. These designs are not new. The basic format was introduced at the end of 2017.
But what is new is the way your site-owners can start using them. Now site-owners can easily apply site designs from the user interface. Which makes it very easy and user-friendly. Enabling this function was part of this roadmap item.