To fold in ingredients is a term typically used in baking. It usually refers to whipped egg whites or cream that is gently combined with a heavier mixture to lighten the mixture and help it rise.
I use a rubber spatula and begin by mixing in 1 dollop of the egg whites or cream from the spatula with the other mixture. I stir and combine this well. Then I take a small portion at a time and carefully and gently fold or turn the egg whites or cream into the other ingredients until incorporated. Be careful when folding not to deflate the whipped, lighter ingredients. The goal is to maintain volume. Less is more.