Fill Rate: the percentage ratio between Impressions and Unit Requests.

It's the measurement of how many requests are made by your site's player which then results in an Impression - in other words, how many requests find a Campaign in target with your Audience.

The Fill Rate is therefore an indicator of the traffic quality on your site.

It is a very important parameter for evaluating performance as a Publisher, and in particular, the effectiveness of the Ad Units.

For Content Units, it makes more sense to monitor the Activation Rate.