Page CTR is your page Click Through Ratio [that is]... how many people clicked through to your page on your ad divided by the total number of page views.
Page CTR Additionally stated as:
Total number of sales is divided by the total number of page views in order to calculate how successful the site is at turning visitors into customers
For example; on this page a number of vistors will see an ad on the left side panel and click on it to explore further. Some people will see there are ads and ignore them and some people will read the content here and not even notice that there are ads on this page.
Page CTR is the ratio of responses [clicks on the ads] to all page visitations.