Thanks, Robert!
Nutritional yeast (some of us call it “nooch”) is a deactivated yeast, usually comes in flakes. I buy the Braggs brand most often. It has a cheesy, nutty, creamy flavor. Many dairy-free folks and/or vegans use it as a cheese substitute. In addition to things like this sauce, I love it on popcorn.
I should probably mention there are health benefits in it, too. It’s rich in B vitamins and some brands tout it as high in B12, which is hard to obtain for vegans. Good stuff!