The 3D action on Oral-B electric toothbrushes means that they oscillate, rotate and pulsate to give you the best possible cleaning, removing up to 300% more plaque along the gum line than a regular toothbrush. It works by pulsating to break up the plaque, and then oscillating and rotating to sweep the plaque away.
The Oral-B Vitality and DB40x0 series use a slightly inferior 2D oscillating rotating mechanism of action. This means that it only oscillates and rotates instead of oscillating, rotating and pulsating. The 2D mechanism of action will clean your teeth better than a regular manual toothbrush however not as well as the 3D.