This is a WebMention endpoint.
You likely came here by clicking a link from a site and that likely means that the site you came from is accepting webmentions.
If you have written something on your own site about the site you came from (or are using eg. brid.gy), then you can make that site aware of that by sending it a WebMention.
You should not send that sites webmentions directly here, but rather use proper discovery of the other site's endpoint as that might change in the future.
Sounds interesting? Want to accept webmentions yourself? Sign up to this service or host one yourself – there are many plugins out there for popular tools like WordPress.