You would have heard about this over and over again. Google always shows related ads. But at some point all of us face the problem where the appropriate ads are not shown. When this happens you can both write to Google and tell them about it or you can implement section targeting.

Section targeting allows you to suggest sections of your text and HTML content that you’d like us to emphasize or downplay when matching ads to your site’s content. By providing Google with your suggestions, you ll improve your ad targeting. We recommend that only those familiar with HTML attempt to
implement section targeting.

CTR stands for ‘Click Through Rate’, that is, the percentage of users who click on an ad on a page they are viewing. It is the second most important thing after Traffic. Lets say, you have an average of 100 unique visitors to your website and your CTR to be 10%, i.e. 1 in every 10 user visiting your website would click on an ad. If your average earning per click is $0.5, then assuming that my calculations are right, $5 in a day from those 100 visitors.