GAM Backfill Coverage is a Metric in Google Analytics under the DoubleClick for Publishers Backfill section
GAM Backfill Coverage Definition: Backfill Coverage is the percentage of backfill ad requests that returned at least one ad. Generally, coverage can help identify sites where the publisher account isn’t able to provide targeted ads. (Ad Impressions / Total Ad Requests) * 100. If both AdSense and Ad Exchange are providing backfill ads, this metric is the weighted average of the two backfill accounts (DFP linking enabled and Hide DFP Revenue not enabled).