This means you can now explore and "try out" in a shared playground (*) the new API features that have been added to Community 201707 (5.2.g) and ACS 5.2.1. These include Group Management, Downloads (as a Zip), Reset Password & improvements to the Search APIs.
(*) Please be aware that (as always) this is a publicly accessible & shared demo server. Please do not upload private, sensitive or valuable data. The server is not backed-up and may be reset or taken offline without warning.