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.
Read the documentation for more details on using Microsub and Aperture.