If the victorious unit charged into combat and the enemy was wiped out, it can move 2D6 inches 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 or combat result (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.
If you charge an enemy and they are wiped out before the Close Combat phase (by a spell or template weapon for instance), you can choose to either Overrun or Reform From Victory (unless the unit has made any other moves this turn outside of the Movement phase). This occurs at the start of the Close Combat phase before any blows have been struck.