Aperture is a Microsub server. 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.
Another piece of the IndieWeb puzzle. #indieweb