Monocle is a Microsub client. Microsub is a spec that provides a standardized way for reader apps to interact with feeds. By splitting feed parsing and displaying posts into separate parts, a reader app can focus on presenting posts to the user instead of also having to parse feeds. A Microsub server manages the list of people you're following and collects their posts, and a Micropub app shows the posts to the user by fetching them from the server.
To use Monocle, you need to have a website which points to a Microsub server so that Monocle knows where to find content to display.
If your website also supports Micropub, then you'll be able to favorite and reply to posts from within Monocle too.
You can read more about the technical details of Monocle at Building an IndieWeb Reader.