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 Microsub app shows the posts to the user by fetching them from the server.
Aperture is just one option for getting started in this ecosystem. You can find a list of other Microsub servers on indieweb.org, many of which are able to be self-hosted.
If you're so inclined, you can even write your own! As long as your server supports the Microsub spec, you should be able to use any existing Microsub app with it!
Read the documentation for more details on using Microsub and Aperture.