Backfill are “catch all” ads shown when nothing else matches and you would otherwise have a blank ad space. These are impressions that none of your partners are buying, or that occur as the result of a misconfiguration in your ad server (such as failing to duplicate creatives). In a perfect world, with a perfectly attentive ad operations team, backfill would not be necessary. All your inventory would be directly monetized. However, reality is messy and ad operations can quickly get confusing as operations scale.
There are a few ways to arrange for backfill.
First, if using DFP, you can enabled AdSense dynamic allocation at the inventory level. We don’t love this approach because it makes things tricky if you ever need to show non-Google ads on pages that violate AdSense policies. By enabling ads at the inventory level, you can’t turn them off on just a few specific pages (there are ways to work around this, but we recommend using AdX at the Line Item level, instead of adopting workarounds).
Second, you can run a 100% fill ad network as House ads. These run with the lowest priority, allowing your higher CPM ads to display whenever possible. Traffic AdX or AdSense as a “3rd party creative”, or just run 100% fill creatives from another ad network. The key here is 100% fill. This layer is the last resort. You don’t want passbacks here.
Finally, a great option is to run true House ads with creatives promoting your own site. This is especially useful if you sell products or services, but can also be used to direct readers to other parts of your content-driven site. You can design your own banner ads and upload the images into DFP. Just remember to account for these correctly in reporting.