Passback or default tags can be any network or remnant tags that you use to fill inventory. When your primary ad network doesn’t have anything to serve (for example, there isn’t an advertiser willing to pay a high enough CPM), you can send that impression back to your default advertiser’s tags to serve. This will allow you to monetize every impression, and greatly reduce the chance of serving blank ads. Passback tags allow you to implement a so-called waterfall model, which allows showing another banner in case the previous one has not been sold via network, RTB, SSP, or Exchange.
Example Passback Tag: