Currently, the extension does a query to the api with prop=description. However, the prop=description is not recognized in a normal MediaWiki installation, which actually gives a warning.
On WMF it works because it's using the WikiData extension. However, most MediaWiki installations out there won't have a WikiData repository.
Instead, there are other extensions that provide a description based on the content of the actual page, and for optimal use outside of WMF, this extension should provide a way to either customize the full API URL, or a way to tell from where to pick the description of the page. Those are alternatives: