Here's a good video explaining them:

I think he has some setting set on Amazon that requires Multi Factor Authentication, I have this on my Wordpress site.
I'm able to log in to Amazon just by using my YubiKey.
VicRoads (DMV) has removed passwords and I just use my YubiKey to log in 

I have 2 Yubikeys that I bought maybe 6+ months ago but have yet to use them. I think that I just forgot hat I had them.  

I do plan to start using them soon.