It’s when the kernel (or drivers) run into a bug. Also known as blue screen of death on the windows side.

Then userland programs triggering kernel panics are good, since they make your kernels more bugfree and secure, aren't they?

They are good when you are stress testing your setup. They are bad if you are just trying to browse the damned website

How often you find kernel bugs that make your setup actually safer while browsing, and while stress testing?

I never seen that on a Mac though.

Ok. Well, it does happen. Happened a lot more in the past vs now.

They are more rare one current versions of macOS. I’ve been using Mac’s for about 20 years and got them a lot a lot in the early days.

They used to happen all the time with Mac OS 9 and earlier, but I don’t think I’ve ever seen them with OSX/macOS.

I definitely had a corrupted CD-R backup that would kernel panic OS X, Windows 2000, and Linux, circa 2003.

Though, most of my OS X kernel panics were due to a GPU slowly going bad and randomly corrupting memory (both under OS X and Linux, when in a particular graphics mode).

QNX is the only OS that I've pushed hard and never seen a kernel panic. BeOS was nice in that my (userspace) ethernet driver would crash overnight most nights, and I'd wake up to a prompt saying "I'm going to restart the crashed driver? Okay?", but I could reliably kernel panic BeOS with some dodgy semaphore code in userspace.

It happens on Mac. It's when you get an infinite spinning beachball. Sometimes called beachball of death, but generally called kernel panic on Unix systems.

Most of the time with Macs it starts happening when you have hardware failure. It happens way more on Windows and Linux because of the much larger range of supported devices and drivers and the varying quality of the drivers for them. Most drivers running in Darwin as made by Apple. It's also the reason Microsoft created a certification program for device drivers for Windows.

A kernel panic is, when your computer presses the panic button and stops to work.