Ad Networks don’t buy 100% of your traffic. That’s why you need to set up backurls to avoid wasting any unsold traffic that has been returned from the ad networks. Copy your universal backurl from the section 'Ad Networks' and set them up for every ad network that needs it.
See the instruction in step 1 of our guide to setting up backurls here.
The Fallback url is used for unsold, cookieless traffic that has been returned from the ad network. To set this up, follow the instructions on the top of the page in the Campaigns section and follow the advice on 'Step 5. Fallback URL Setup'.