To understand what “Consumption” means, you must first understand the “Stories Created” metric. A brief description of this metric: Facebook considers a ‘story’ to be any quote, comment, check-in, like, rating, or post from the page that appears in the users’ timeline (News Feed). So story is a very relevant engagement metric as it represents a brand display on the users timeline.
The “Consumption” is the number of times an user interacted with the page (like, comment, share…) but did not created a “story”, that is, didn’t appeared on the other users News Feed.