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.