Advertising. A lot of websites contract with DoubleClick (which is owned by Google) to place their ads on other websites. This allows DoubleClick to both serve the ad and to track the delivery and charges for advertising.
Your anti-malware software might be blocking the DoubleClick.net domain as a policy of blocking spyware or advertising tracking sites, hence you are not able to access it. Or it might have placed an entry for this URL in the HOSTS file.
Anyone can block all access to this domain on their computer with the HOSTS file.
This website keeps track (no pun intended) of advertising websites that use tracking cookies and publishes a HOST file that helps you block those sites.
But the instructions on the website are terribly written, it's no problem for me, as I know what to do and ignore the instructions. Otherwise, the information is useful, if obfuscated by other details.