But ultimately, a Webmention is no more than a link from post A to post B, a POST request and a markup that tells what post B actually is about.
A bit more deeply…
Let me start with the Webmention protocol itself. This is a way to notify a site that another page exists that links to the site, and to find information that may be relevant. An example.
Suppose Adriaan and Bassie both have a blog that supports Webmention. Adriaan wrote a piece and posted it on his blog. Bassie reads that and has an opinion about it. To express this opinion, Bassie post a cat picture on his own blog, indicating that it is an answer to Adriaans piece.
As soon as Bassie posts the cat picture, Bassie’s server sends a notification (a POST request) to Adriaans server, containing the link of Bassie’s post and Adriaans piece. Because Bassie has indicated that it is a reaction, there is a link in his section a link to Adriaans piece. Adriaans server looks at the page, sees the link to their own site, and then accepts the Webmention by returning ‘OK’. (More precisely, an HTTP 200, 201 or 202.)