Webmentions are a way for websites to interact. Most commonly, it’s used for comments, likes etc. on the IndieWeb. It basically works like this:

Webmentions can be sent in many ways: automatically by the website’s own server, manually by person B, or by an external service like https://webmention.app.

They can also be received by external services, like https://webmention.io. This is useful for static websites without a server to receive and store them.

If you don’t have a website, you can send webmentions using https://commentpara.de.