Pleroma is a microblogging server software that can federate (= exchange messages with) other servers that support ActivityPub. What that means is that you can host a server for yourself or your friends and stay in control of your online identity, but still exchange messages with people on larger servers. Pleroma will federate with all servers that implement ActivityPub, like Friendica, GNU Social, Hubzilla, Mastodon, Misskey, Peertube, and Pixelfed.
This is the source to the instance running at thefloweringash.com, please see the upstream project on git.pleroma.social.