What is an "App Password"?

[freeradical]

I had to get one and use it to login to my Fastmail account via the Mail app with my new Mac Mini. I've never had to use anything like this before. I use the Mail app on my MBP without using one of these weird passwords. It was a 16 digit alphanumeric string, just like a key to activate M$ office or something. I didn't write this code down. Was this an oops?