In reply to Flancia!.

For a base introduction I've written a piece for A List Apart. For the more technical side, I recommend the IndieWeb wiki:

Webmentions page: https://indieweb.org/Webmention
Webmentions developer page: https://indieweb.org/Webmention-developer

There should be lots of links to open source projects with code in a variety of languages already, so don't reinvent the wheel if you don't need to.

For framing purposes, Webmention is about sending/receiving notifications, but if your site has proper microformats markup and the receiving site has a parser, then it's not too difficult for the receiving site to copy, store, and show the notification as a comment or reply. This should allow wikis/digital gardens/commonplace books to "chat" with each other and even send updates (or deletes) for content which changes.

If you have questions or run into problems there are many in the IndieWeb #Dev chat channel who have implemented them and are happy to help answer questions.