Any concept should be explainable to a child. If it is not, then the concept is not fully understood.
Applied to note taking, the notes can be written like they are addressed to a child. By reading the notes (reviewing) the gaps in knowledge can be found. After filling in the gaps, the note can be changed until a concept is explained in a simple way with plain language.
Think of questions a child might have. Why is a concept like that? This helps to test if a concept is fully understood.
"Feynman" technique is named after Richard Feynman. A Nobel Prize winning theoretical physicist.