The Atlassian REST API Browser (RAB) is a tool for discovering the REST APIs and other remote APIs available in Atlassian applications, including JIRA. It shows the available REST resources, the parameters required for them, and so on, and allows us to make a test call using valid input data. The RAB will return the response, as appropriate.
Most importantly, the RAB shows both internal JIRA APIs and external APIs exposed by third-party plugins. This is very useful as most of the methods exposed by third-party plugins and even some of the JIRA internal methods don't have proper documentation.
If you are running the Atlassian plugin SDK to develop a plugin, the RAB is already installed in the JIRA instance spun up by the SDK. If not, you can get the plugin from Atlassian Marketplace.
Once the RAB is installed in JIRA, you can navigate to Administration>Add-ons>REST API Browser. Clicking on the REST API Browser link will open up the RAB screen as...