This feature allows you to select multiple icons/items at once and move them all as a group. To do this, simply hold down the shift key and then click on which icons you would like to move. One selected, the icons will be highlighted as shown above. You can then either drag the icons around, or nudge them with the arrow keys on your keyboard.
While dragging icons around is quicker, nudging the icons by small increments at a time allows you to be more precise with the placement of each icon.