GAM Backfill Monetized Pageviews is a Metric in Google Analytics under the DoubleClick for Publishers Backfill section
GAM Backfill Monetized Pageviews Definition: This measures the total number of pageviews on the property that were shown with at least one ad from the linked backfill account(s). Note that a single monetized pageview can include multiple ad impressions (DFP linking enabled and Hide DFP Revenue not enabled).
Data Analytics & Insights Google Analytics Metrics GAM Backfill Monetized Pageviews