The full name is "autonomous sensory meridian response." And in my case, it comes to pass when I listen to certain sounds in certain circumstances. When the conditions are just right, I experience a delightful sensation somewhere in the back of my neck, but also sort of deep in my brain. The magic formula can involve crinkling paper, or whispering, or the tapping of fingernails. It also has something to do with where the crinkler, whisperer, or tapper is focusing his or her attention.
I know: weird, right? But it's oh-so-real. See that colored-in illustration and misspelled explanation above? I made that when I was about 10 years old. My very thoughtful mother even bought me a ream of onion-skin paper when I was a kid because she knew I loved the noise it made.
It's a little embarrassing to share this information, but I'm certain that I'm not alone. I even know a couple of other people who experience ASMR. (Interestingly, they both have synesthesia.)