The Visibility Score feature is a core metric which assists in highlighting data issues and/or optimisation opportunities. Opportunities that are fundamental to any digital marketeer who strives to improve their data performance.
In a Google Product Feed, within the Performance UI you'll now have access to a Visibility Score window. 
WHAT IT'S USED FOR
The Visibility Score shows how your products have performed in two ways:
- the average score for the Partners set timeframe (in this example 15 days) vs the previous timeframe before that
- and the latest (last 24 hour period) vs the set timeframe
So the left side of the panel is the last 15 day average vs the previous 15 days before that and will display the percentage of products which were visible during this period, whether the score has gone up or down when compared to the previous 15 day period, and the percentage thereof, and then the volume of products which were visible during that time period.
The right side shows the latest visibility score, the past 24 hours vs the past 15 days. So in the example above it shows all products were visible, so up 7% from the 93% average of the past 15 days. Keep in mind though that while all products were visible (so, obviously a good thing!) but that's just in this past 24 hours. A 15 day period can peak and trough more.