Bridgy Fed turns your web site into its own fediverse account, visible in Mastodon and beyond. You can post, reply, like, repost, and follow fediverse accounts by posting on your site with microformats2 and sending webmentions. Bridgy Fed translates those posts to fediverse protocols like ActivityPub and OStatus, and sends fediverse interactions back to your site as webmentions.
This isn't syndication or POSSE! You don't need an account on Mastodon or anywhere else. Bridgy Fed lets your site act like a first class member of the fediverse. People there will see your posts directly from your site, and vice versa.
Bridgy Fed takes some technical know-how to set up, and there are simpler (but less powerful) alternatives. If you just want your site's posts to show up in the fediverse, without any other interactions, consider an RSS or Atom feed bot instead. Or, if you want to cross-post to an existing Mastodon account, try Bridgy.