If one model slays the other, then any excess wounds they inflicted above and beyond those needed to slay the opponent, up to a maximum of +5, are counted towards their side's total number of wounds for close combat result. More on this bonus can be found in the Close Combat chapter.
Note that this is an exception to the rule stating that a model can only suffer as many wounds as it has on its profile. This time you need to add up all of the wounds inflicted on the victim, even those from a weapon causing multiple wounds, or by repeated Killing Blows (each successful Killing Blow scores the same amount of wounds the slain character has on its profile), etc. This is great fun, albeit a little one-sided.
For example: A Chaos Lord fights a champion in a challenge. He slays the champion before he has a chance to attack and causes seven wounds! The combat result score is 1 (the original wound of the champion) plus 5 (the maximum overkill bonus) for a total of 6 combat result points. The last of the seven wounds is wasted.