From the introduction:
A Webmention is a notification that one URL links to another. For example, Alice writes an interesting post on her blog. Bob then writes a response to her post on his own site, linking back to Alice’s original post. Bob’s publishing software sends a Webmention to Alice notifying that her article was replied to, and Alice’s software can show that reply as a comment on the original post.
Sending a Webmention is not limited to blog posts, and can be used for additional kinds of content and responses as well. For example, a response can be an RSVP to an event, an indication that someone “likes” another post, a “bookmark” of another post, and many others. Webmention enables these interactions to happen across different websites, enabling a distributed social web.