If the victorious unit charged into combat and the enemy was wiped out, it can move 2D6" straight forward.
If a unit charges into combat and, by the end of that round of close combat, all its enemies have been wiped out as the result of the combat (rather than the pursuit) the unit can choose to make a pursuit move, even with nobody left alive to pursue. This is an overrun move and represents the unit surging forwards, hungry to find more enemies to fight.
An overrun is essentially a special pursuit move. When making an overrun, the victorious unit moves 2D6" directly forwards, as if they were pursuing a fleeing enemy to their front. All other rules governing pursuit moves, such as intervening units and terrain, apply to overruns.