Good question! Amazon defines a “low-content book” as a book that “has minimal or no content on the interior pages.”
Here is their official statement, which (if you’re curious) was just issued in May of 2022:
This is a little weird for Amazon to say, because technically they do not allow truly blank books to be published on their KDP platform.
Low-content books are things like notebooks, planners, sketchbooks, logbooks, trackers, and journals.