The On WebSocket Start Node is an Event Listener Node notifying that the WebSocket connection was successfully established, therefore enabling the user to perform actions on said connection.
WebSocket Communication in Incari is available as a plugin and is enabled as default. However, in the case that it is disabled in the Plugins Editor, it will not appear in the Project Settings and On WebSocket Start will not show up in the Toolbox. Please refer to the Plugins Editor to find out more information.