This is the Drupal 8 version of Page Manager, formerly part of the CTools module written by merlinofchaos It supports the creation of new pages, and allows placing blocks within that page. Like Drupal 7's Page Manager, it provides a concept of "page variants", each with their own selection conditions. Additionally, it can be used to take over an existing page, like overriding /node/% to change what is displayed when viewing a node.