Ever do something on the web, like shop for shoes, and then notice that every other website you visit has ads for shoes? That’s the result of website tracking. It’s a little creepy, the idea that you’re essentially being followed on the web, targeted with advertising. (And yes, Macworld.com is guilty as charged.)
Apple’s Safari uses WebKit as its engine to present websites through a browser window. WebKit has features to reduce the amount of tracking a site does, and the latest feature is called Intelligent Tracking Prevention. ITP cuts down on the ability of a site to do cross-site tracking, which leads to the experience I mentioned above.