Coverage is the percentage of ad requests that returned at least one ad. Generally, coverage can help you identify sites where AdSense isn't able to provide targeted ads.
Coverage = (Ad requests that returned ads / total ad requests) * 100
For example, if you have 3 ad units on a page, you'll generate 3 ad requests. If two of these ad units display ads and one displays no ads, the coverage for this page would be 66.67%. Likewise, if you have a search box, coverage of 80% means that an average of 1 query out of 5 shows no ads with the search results.
High coverage, close to 100%, indicates that Google was able to provide ads to most requests. Lower coverage is a sign that Google wasn't able to provide ads suitable for your page and returned no ads or your specified backup ads instead. To maximize your coverage, we recommend reviewing the possible causes for low coverage.