Pure Beginner's Question

[RossA]

I am not a knifemaker, although I love good knives. I have read different forums, blogs, etc. about knifemaking and heat treating. I was always under the impression (quite possibly wrong) that after a blade was almost fully formed, it was hardened at a relatively high temperature and quenched, then tempered at a lower temperature to reduce some of the hardness and impart some toughness to the steel. I know that this is very oversimplified.
I have been watching the TV show Forged in Fire, like probably some of you have. On that show, after they forge blades, they heat them and quench them one time, without a secondary heating and quenching.
Help me understand the one step vs. two step heat treat. Thanks in advance.