Sling provides a way to automatically create a resource name. If you send a POST request to an URL ending with a /*, Sling will generate the resource name, using a number of rules described in the documentation.
If you intercept the request sent to the /* resource in servlet or filter and invoke the request.getResource() method, you'll get a StarResource object. It extends the SyntheticResource type, representing resources that doesn't have appropriate node in the JCR. Its resource type is sling:syntheticStarResource, you can call its getParent() method, but generally it isn't very useful. An attempt to adapting it to ValueMap will fail. Sling provides a helper method that checks if the given Resource is a StarResource.
In case of the paragraph system implementation mentioned in the question, the author of the code probably wanted to display the Drop component using a synthetic resource and providing * as the path is the easiest way to do it. You can also be sure that the resource will always be synthetic, as it's impossible to create a node with this name.
