Not all of the attacks that do hit will harm your enemy. They may be deflected by equipment, parried at the last moment or merely inflict a graze or flesh wound. Once you have scored a hit with an attack you must roll again to see if you score a wound and incapacitate your foe.
The procedure is the same as for shooting. Consult the chart, cross-referencing the attacker's Strength characteristic (S) with the defender's Toughness (T). The chart indicates the minimum D6 roll required to inflict a wound.
Example: A Space Marine (Strength 4) hits a Dark Eldar (Toughness 3). Referring to the To Wound chart below we find that the Space Marine requires a 3 or more on a D6 to Wound.