The Coverage Rate refers to the number of ads visible in comparison to the total number of ads present on the results page, multiplied by 100.
Myposeo analyses a keyword 10 times, and your site is present twice. On these 8 times, your site normally has only 1 Adwords ad.
The Total Coverage Rate is the Average of all Coverage Rates for the time where your site is present on the results page, which is 2 in our example. The Coverage Rate is therefore 42%.