Much like other terms that refer to a person's unwavering dedication to a particular subject a yarnie is a person how is obsessed with yarn. This can include making things with yarn, making the yarn itself and dyeing the yarn.
Some of us are insanely creative while others simple enjoy collecting yarn.
My personal yarn journey started when I was in the military. I needed a hobby that was inexpensive and traveled well. Knitting with yarn was a perfect fit.
As my knowledge grew I found out about fine fiber yarns, yarn dyeing, and even yarn spinning. It was and still is a never ending rabbit hole of creativity!
I currently own enough yarn and wool to survive a zombie apocalypse or make enough sweaters for my family for the next 5 life times.
So next time you see someone working with yarn. Ask them what kind it is. You may learn something amazing and maybe even spark your inner yarnie.