This is the process of combing two fluid substances of different thicknesses together my inserting a spoon into the bottom of the heavier mixture, pulling some to the top and over the lighter mixture.
I found an article which explained it:
Folding is a very precise term in cooking and baking. It means that you have to carefully combine two mixtures of different thickness and weight into one (relatively) smooth mixture. This is accomplished by a specific technique of using a spoon to lift the two mixtures together, turning them over so they combine.
This is apparently the reasoning:
This traps air into bubbles in the product, allowing baked goods to rise.
Why I Looked It Up
I had heard the term for years, but assumed it just meant “blend.” I started to run across it more and more when I subscribing to Hello Fresh food boxes in 2020-21.
There was also a joke about this in an episode of Schitt’s Creek when David questioned the meaning of the term to Moira, his mother.
Moira: The next step is to fold in the cheese.
David: What does that mean? What does “fold in the cheese” mean?
Moira: You fold it in.
David: I understand that, but how do you fold it? Do you fold it in half like a piece of paper and drop it in the pot or what do you do?
Moira: David, I cannot show you everything.
David: Okay, but can you show me one thing?
Moira: This is what you do. You just fold it in.
David: Okay, I don’t know how to “fold” broken cheese like that.
Moira: I don’t know how to be any clearer! You take that thing that’s in your hand, and you –
David: If you say “fold in” one more time…
Moira [holding out recipe]: It says “fold it in”!
David [taking off apron and leaving]: This is your recipe! You fold in the cheese then! You fold it in!