WebFinger is used to find the ActivityPub profile for your site.
References
Configuration
WebFinger requires a hostname and username to generate the profile.
Configuration alternatives on _config.yml:
activity_pub:
hostname: "example.org"
username: "me"
Will become an account @me@example.org.
hostname: "example.org"
# or
url: "https://example.org"
Will become @example@example.org.
If you use a CNAME file, this plugin will find the hostname there, so you don’t need to repeat the configuration.