fold in?

When a baking recipe tells me to "fold" something in, what does that mean? How do I do it and why?

(I know I'm a moron, I appreciate your tolerance )

I haven't made cakes or cookies in a while but to me Fold in means to gently mix something into the batter. What comes to my mind is using a plastic spatula to gently add vanilla, and using the spatula to fold the batter over top of itself.

folding is a gentle way of mixing a lighter ingredient (such as beaten egg whites or melted chocolate or melted butter) into a denser/heavier ingredient (cake batter, for instance) - ingredients are "folded in" (beaten egg whites for instance) so as to gently mix them in without losing their volume - when folding you want to trap that extra air in the batter -

folding is usually done after the main ingredients of the recipe have been blended together -the lighter mixture is placed on top of the heavier one in a large bowl. Using a clean and dry rubber spatula - start at the back of the bowl, and cut down vertically through the two mixtures, across the bottom of the bowl and up the nearest side; rotate the bowl a quarter turn with each series of strokes. This down-across-up-and-over motion gently turns the mixtures over on top of each other, combining them in the process.

I hope this helps

Found this one:
'To mix in or add (an ingredient) by gently turning one part over another
For example, Fold in the egg whites.'

I think we do this on making tacos too.