It's just a short video from Alex Wheeler who explains it far better than I could, but it's a sharp click that occurs at the top of your bolt lift where the action's primary extraction occurs and meets resistance due to the base of the fired case being too tight in the chamber. They can be mild to quite stubborn/tough to extract. It isn't necessarily the die - it could be a chamber that is too tight as well. The 300PRC is the poster child for this. Some of the premium dies (Area 419, SAC, Micron) are supposed to size the base more than traditional dies, and help prevent this. I use the Area 419 for 300PRC and haven't experienced the problem that is so common with them. I just recently ran into it with 300NM (Redding die), but neither Area 419 nor SAC make a 300NM die. But, Micron does, and is supposed to size the base smaller, so I'm giving it a try to see if it will help.
Highly recommend you watch the video. He knows far more than I. Well worth 5 minutes if you haven't heard of the problem before.