Fill Rate: the percentage ratio between Impressions and Unit Requests.
It's the measurement of how many requests made by your site's player result in an Impression, are aligned with 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 the performance of your Publisher activity, in particular for the effectiveness of the Ad Units.
For Content Units, it makes more sense to monitor the Activation Rate.