ArcGIS Enterprise Sites allows you to create a tailored web page experience for your users to help you share your portal's authoritative GIS data to other departments more easily, even if they are not used to working in your GIS. Instead of learning to navigate the portal and access groups, members can go directly to the custom web page you create to navigate the content relevant to them.
Web pages created with Sites are built to be easily navigated by non-GIS users, but are powerful enough to let users interact with, download, and create apps from your data. Sites features a built-in web page designer that integrates a drag-and-drop experience with HTML markup, giving you absolute control over the design of the web pages you create. You can also add non-GIS content, such as videos and documents, to your Site to create a central place for material relevant to your users to be hosted and accessed.
You can create as many independent, tailored web pages as you want, and security and access to your Site and content is controlled by the ArcGIS Enterprise portal. Individuals within your organization can be added to your Sites team to gain permission to edit and create web pages using ArcGIS Enterprise Sites.
Within a Site, you can create pages to further organize and focus your content. A Site can have many pages, and these pages can be shared across multiple Sites if needed. How you choose to organize your Sites and pages is up to you and the needs of your organization.
The Sites app can be accessed through the Apps button 
in the header of your portal.
Note:
You can also access the Sites administration page using the URL https://webadaptorhost.domain.com/webadaptorname/apps/sites/admin.