Inventory Fill Rate is a metric that shows the percentage of inventory that was sold.
In an attempt to increase fill rates and to reduce remnant inventory, publishers use multiple ad networks.
See Ad - Waterfalling (daisy chain or waterfall tags)
Ad fill rate is also a parameters on ad exchange/ad network to tell how much of the available ad space should be filled on the site. For example, a 70% ad fill rate means that ad units will be filled up to 70% of the time.