Troops lining up behind a wall, hedge or other obstacle can adopt a position to defend it. The front rank is moved right up against the obstacle to show this. Enemy wishing to attack the defenders can do so by charging them as normal. Attacking models don't have to physically cross the obstacle, indeed they are unable to do so while it is defended. Instead, the front rank is positioned on the opposite side to the defenders.
Models charging an enemy behind a defended obstacle do not get any of the bonuses normally associated with a charge and instead fight as if they were already engaged in the combat from a previous turn. This means they do not automatically strike first, but strike in Initiative order, do not get any Strength bonus from lances and other weapons that confer bonuses to the charger, no special rules that rely on them charging would work, and so on.
Flying models attacking enemy behind defended obstacles ignore the penalty.