There is extensions to web browsers like IE, Firefox, and Chrome that remove the ads from the web page when it is shown. These Ad blockers were developed to solve a real issue. Many sites had intrusive ads and even worse some scammers were using ads to deliver ransomware and other malware. The ad serving companies seemed unable to stop the scammers from using their ads.
The ad blocking software protected its users from being bothered by intrusive ads and possibly receiving malware. Adblock is the most popular of these extensions and claims 40 million users.
The downside of this software is that many of the sites on the Internet depend on ads to support them. Some sites Forbes, Wired, etc. have popups that request that you allow ads from them in order to support them. For a while Forbes would not allow users with adblock to enter the site.
Adblock has a list of requirements for acceptable ads. The site has to comply with these requirements and submit a request to not have the ads blocked. Adblock may decide to allow that site’s ads to be seen.
The ad blocking software is the result of past problems with web ads. The ad blocking is causing sites to have to come up with a new revenue model or find a way to have their ads allowed.