It's the total engagement value attributed to the asset that occurs after site visitors have been exposed to the asset, divided by the number of visits to (exposure to) the asset.
The value is collected against all goals that have an engagement value number, not just goals that specifically assigned to an asset (directly, or via campaigns). Example: If you have a 5 point goal for asset download, a visitor would accumulate 5 points of engagement value for downloading the asset. If that same visitor went on to fill out a contact form that had a 25 point goal, they'd accumulate 30 engagement points for that session. And, 30 points would be attributed to that asset.
The intent is to show assets that influence (directly or indirectly) positive visitor behavior.