guestbook is a way to allow visitors to your site to leave a comment about the website as a whole, instead of a comment about a specific post.
In the context of IndieWebCamps, starting with IndieWebCamp Baltimore 2018, the term has been used for the page that attendees can optionally share the projects they're working on. See: 2018/Baltimore/Guest_Book
Community: A guestbook lets you hear from the people who visit your site. Visitors can leave a comment about anything, whether it's the structure of your site, its ease of use, or something interesting they have discovered. The feedback visitors leave could be valuable in helping a webmaster improve their website.
Nostalgia: Guestbooks were a common feature on personal websites in the late 90s and early 2000s. Adding a guestbook to your site is an ode to the web of the past.
A guestbook can be set up using webmentions. A website should have a devoted page for guestbook entries to which users can send a webmention.
A website may, optionally, provide a form through which people can send webmentions.
- jamesgoca uses Webmention.io to retrieve webmentions submitted to his guestbook.