Webhooks are HTTP calls to an external server that pass some data. The receiving side can process that data and perform an action: store it, run an action, and so on.
Webhooks are configured on the tab of the same name in Automatic Actions, so they can be called on events related to creating or changing an artifact, on a schedule, and so on.
Besides the URL (the address the request is sent to), you need to build the request body. The body is usually defined by the receiving side's API. You can build any body using session data (user, event type, and event date) and data about the artifact that produced the event.
The hint for the message field lists substitutions that are replaced with values when the request is sent to the specified URL.
You can view the log of completed requests in the Administration section, in the "Webhooks" module. Use this log to debug calls to external APIs and to analyze problems with sending and processing requests.