Fingerprinting is using some aspect of your computer’s hardware to determine a unique but stable identification pattern for it. It can then be used to track you online. In this case, the way an audio oscillator in every computer generates a signal can be used to uniquely identify that computer. This is called audiocontext fingerprinting and more details on how it works are here.
Audiocontext fingerprinting is broadly used and growing quickly. An older list of sites researchers found using audiocontext fingerprinting includes Expedia, Hotels.com, Travelocity, Chegg, LiveScience, and many others. That list sadly would be much longer today.
Contrary to what Lifehacker reported, there is a tool you can use that protects you from audiocontext fingerprinting: Epic! Epic now blocks audiocontext fingerprinting (in addition to many other fingerprinting techniques). You can test that here.
Protect yourself from tracking. Use Epic!