What is an Apple ID recovery key?

Someone went to the new authentication system, and got locked out of his Apple ID. He went to Apple, but they said he needed his Apple ID recovery key. He lost everything in iCloud and iTunes.

What is that? I've never heard of such a thing.

I had an Apple account before there was an Apple ID, and probably got my Apple ID in 2005. While I have been forced to use the persnickety 2 step authentication system, I found out how stupid a mess that was when my Android phone got stolen and I was locked out of iCloud until I could get a new phone. Because of iCloud, I was UNABLE to get a new phone number.

No one has ever mentioned a "recovery key".

If I ever got one, it was 3 computers and almost 2 decades ago.

What do I do now?