These are screen in the SSMS/25Live that you can embedded into your webpage. The primary use of these is Location or Resource availability calendars. Since these are not events based, they provide information that cannot be added to other web calendars such as hours and blackouts.
To add a calendar embed to your webpage you will first need to request the Embed Calendar be created. The best way to determine what configuration you would like for the calendar embed is to look at the screens look like in the SSMS/25Live. Go to the Location, Location Search, Resource or Resource Search and click on the different view tabs (Availability, Availability Weekly & etc.). What you see on screens what you will see for the embed. This includes the filter and navigation options, see the example below. Once the Embed has been created you can add it to your webpage using the 25Live Embed snippet.