That is just telling you that the "Sidebar" page is under the "MediaWiki" namespace on your installation.
You navigate to that page the same way you do to any other MediaWiki page:
In this case, the pagename also includes a "namespace", so the urls would be:
To illustrate this further, check out your "search" page, and note that numerous name spaces available - You will see that MediaWiki is one of them.
You can also "find" that page on your wiki by searching for "Sidebar" with only the "MediaWiki" namespace selected to be searched.