Viewability and fill rate are two of the most commonly cited ad metrics, and they measure very different things: one is about whether an ad was actually seen, the other about whether an ad request was answered at all.
Viewability
Viewability measures whether an ad impression met a minimum visibility threshold — typically a set percentage of the ad's pixels on screen for a minimum duration. An ad that loads but scrolls past instantly, or renders off-screen, technically served but may not count as "viewable," which matters because advertisers increasingly pay only for viewable impressions.
Fill rate
Fill rate measures the percentage of ad requests that are successfully answered with an ad, out of all requests made. Low fill rate means an app is asking for ads more often than advertisers are available to serve them, leaving impressions — and revenue — on the table. Mediation platforms exist largely to push fill rate as close to 100% as possible by pulling from multiple demand sources.