This page provides information about the Alfresco ReST API Explorer, how to install it and use it.
The Alfresco ReST API v1 is described in the OpenAPI specification format (formerly Swagger specification). This is good as lots of different tools can read this format, and you can also use the specification to generate ReST API clients in different languages, such as Java.
Alfresco uses the Alfresco ReST API OpenAPI specification to provide an API Explorer, so you can easily read about all the APIs and try them out. The API Explorer comes in the format of a WAR file that needs to be installed (dropped) into your ACS installation.
If you just want to have a look at the latest API Explorer, then you can use an online version of it available here: https://api-explorer.alfresco.com/api-explorer/