Simply means movements that God designed your body to perform naturally. A squat is a natural or functional movement. You squat every time you sit in a chair. Strapping yourself in a leg extension machine and extending your legs with 100lbs on them is not natural and not functional. Functional movements are safer and reduce opportunities for injuries. Each time you pick up a child your deadlift. Each time you place your suite case in the overhead compartment you are doing a snatch.