Basically, this video talks about creating an arbitrary (and almost a placebo) clipping point for you to mix to, with the peak levels being much lower than 0DB so you can get a cleaner mix, and a more worthwhile master. The whole "K-meter" thing doesn't really serve a big purpose, it's just a tool to help you keep your levels low and clean. (Even though I know it's better to mix at low volumes, I still tend to push everything a lot higher, because I'm an idiot) but if you want to improve your mixing, you should watch this.