Over on the LibreOffice Wiki they describe this new NotebookBar. That page explains, "The goal is to provide an alternative (optional) interface which unifies all the different toolbars and groups them in tabs. It will allow more flexible layout than the toolbars, since Glade will be used to lay out the controls. The classic menu will probably be enabled by default in environments like Unity and Mac OS (global menu). In other environments there might be a switch to enable the classic menu. The first app where the NotebookBar will be introduced is Writer."
The screenshot was included on that page to reflect the current state of the new toolbar. Within the latest LibreOffice Git code, the new toolbar layout is activated by setting the LO_USE_NOTEBOOKBAR=1 environment variable. There's no word yet when the NotebookBar will be ready for day-to-day use and available via all LibreOffice components.