For the first time, Apple will allow adverts to be blocked by the iPhone and iPad versions of Safari. The move is likely to please users, but will concern the many companies that depend on advertising.
In a nutshell, the term covers a variety of technologies used to prevent adverts appearing on internet-connected devices. They are already widely used on PCs, where the most common technique is to install a browser plug-in, but are relatively rare on smartphones and tablets. That's not to say it's impossible to use them on mobile kit.
Apple and Android devices can already run specialised third-party ad-blocking browsers or be made to stop ads appearing by altering their network settings, but the point is that only a small percentage of people do this. Apple's decision to open up Safari, however, could take the activity mainstream.
Webpages should be decluttered of distracting content. Pages should also load more quickly, mobile data allowances should come under less strain and iPhone batteries could also last longer between charges. These browser add-ons can be set to block certain cookies, images, pop-ups and other content from being downloaded.
Until now, the only way to do something similar was to "jailbreak" the handsets, which also made them more vulnerable to malware. Apple will not offer its own ad-blocking software. Instead, people will be able to download extensions made by others from its App Store in a similar way to how they can already install third-party keyboards.
What happens to those who rely on ads to make money?
Absolutely. There have already been loud complaints about the spread of ad-blockers on PCs and several of Germany's leading publishers have tried and failed to block their spread via the courts.
Their fear is that the practice could now become widespread on mobile.
About 198 million people - representing roughly one in 20 Internet users - already use ad-blockers on desktop computers, according to a recent study by Pagefair, a company that sells a workaround to the extensions.
iPhones and iPads may be outsold by Android devices, but they represent "52% of the mobile browsing market and 14% of total web browsing", its report states, adding that Apple's move could be a "game changer".
The Interactive Advertising Bureau (IAB) trade association also warns that the consequences could be calamitous for some sites.
"If you are a medium or small-sized website operating on very tight margins, this could make or break the business," suggests Stephen Chester.
"Particularly news organisations - whose revenues are under fire at the moment as their print circulations diminish but online audiences grow. Those organisations are having to reshape to adapt to the digital world and ultimately this could break them or put them at risk."