The term isn't unique to Amazon, though it has a different meaning when used elsewhere. Inc. Magazine explains:
Plog n. 1. A Web-based tool used by colleagues to keep tabs on group projects. 2. An Amazon trademark, which will presumably be used to sell more books.
Now that we're all familiar with blogs, along comes a new new thing: the plog. Though there are a variety of definitions for the term (which Amazon has trademarked), the most relevant concept for business owners is using blog software to create a password-protected project log -- or plog for short.