When attacked, Models typically roll dice to determine if they avoid the attack. In most cases this is the dice of the Armor(ARM) value. In the case of models with Dodge the Dexterity(DEX) value can be used instead (unless the model is suffering the Slow condition). Some Special Abilites (those that do not inflict Wounds) may indicate that other stats are used for Defence (such as Will).
Defence is successful whenever the defence roll meets or exceeds the Attack Roll's star value. (E.G. the tie goes to the defender). When the Backlash ability is present on the model rolling Defence, exceeding the Attack Roll places a wound on the attacker.