what is bridgy and why is it cool
brid.gy is a cool service that lets you integrate your personal site with social media sites once you've set up webmentions.
Among other stuff, it can find when people are talking about one of your pages on sites like Mastodon and Twitter, and send you webmentions with the content of those posts so that you can display them like comments.
To get all the possible functionality out of Bridgy (like automatically making a post for you on Mastodon or Twitter when you send it a webmention from a new page), you have to mark up your HTML with something called microformats, but you can get reactions and replies on your page without that extra complication.
Check out this example of someone who's set up webmentions and Bridgy!