Electronic Frontier Foundation:

When a site you visit uses browser fingerprinting, it can learn enough information about your browser to uniquely distinguish you from all the other visitors to that site. Browser fingerprinting can be used to track users just as cookies do, but using much more subtle and hard-to-control techniques.

And:

By using browser fingerprinting to piece together information about your browser and your actions online, trackers can covertly identify users over time, track them across websites, and build an advertising profile of them. The information that browser fingerprinting reveals typically includes a mixture of HTTP headers (which are delivered as a normal part of every web request) and properties that can be learned about the browser using JavaScript code: your time zone, system fonts, screen resolution, which plugins you have installed, and what platform your browser is running on.

And:

When stitched together, these individual properties tell a unique story about your browser and the details of your browsing interactions. For instance, yours is likely the only browser on central European time with cookies enabled that has exactly your set of system fonts, screen resolution, plugins, and graphics card.

The linked/quoted article is long and detailed, an enlightening read. But the bits about browser fingerprinting are incredibly important. And this is as good an explanation as I’ve seen.

At WWDC, Apple declared war on browser fingerprinting and related techniques. From Apple’s Mojave press release:

As with all Apple software updates, enhanced privacy and security remain a top priority in macOS Mojave. In Safari, enhanced Intelligent Tracking Prevention helps block social media “Like” or “Share” buttons and comment widgets from tracking users without permission. Safari now also presents simplified system information when users browse the web, preventing them from being tracked based on their system configuration.

And that’s a good thing.