Pinboard is "social bookmarking for introverts"– a social bookmarking web service along the lines of (the now defunct) del.icio.us. pin is a command-line client for Pinboard. With it, you can send links to Pinboard (among other tasks) programmatically, or perform operations that are inconvenient from the web application such as generate a report on your tags.
The author, for instance, uses it, along with Emacs org-protocol, to capture links from the browser and store them simultaneously in his notes and on Pinboard.