It turns your WordPress blog into a Fediverse server, and your WordPress account into a Fediverse account with its own Fediverse address that people can follow.
It means people will be able to follow your WordPress account from Mastodon etc, your blog posts will appear as (very long) posts within Mastodon etc, and people will be able to interact with them as if they were Mastodon etc posts.
I've done a longer form text about it over on the website:
https://fedi.tips/the-fediverse-beyond-mastodon/#WordPress
It isn't crossposting from any other account, you don't need a Mastodon account (or any Fedi account) to use this plugin.