Folding is a technique used to gently incorporate a light, delicate and fluffy ingredient/mixture into a heavier textured ingredient/mixture to make sure they are properly combined without affecting the ability of both mixtures to work as desired. Folding is often concerned with mixing in decisive motions to avoid bursting the air bubbles in lighter ingredient/mixture, thus maintaining the volume of the ingredient/mixture. This technique is often applied in combining flour or whipped egg whites into cake batter.
In general, folding techniques includes these following steps. First, put 1/3 or 1/4 of the light ingredient/mixture on top of the heavier (never do the other way round). Then, with the blade of spatula, cut straight down deep to the center of two mixtures, scoop from the bottom of the bowl and fold over on top. Continue with the other 1/3s or 1/4s and remember to fold in the same direction.