Been a while since I hit one of these. Essentially, you go looking for something online, and the page goes fullscreen and attempts to keep you from closing the tab, often with discomforting audio. In my case I was looking for a speaker teardown manual, and it hit pretty hard. Obvious scam, but it told me it was going to "disable" my computer to protect their network.
I honestly haven't seen poo like this since around 2006, and I'm surprised current browsers still allow that level of hostility from a site.