A model's armour value is determined by the armour it wears. Armour value is presented as a target number, meaning the lower the value, the better the chance of making a successful Armour Save roll. Light armour, for example, grants an armour value of 6+.
A model's armour value can be improved (to a maximum of 2+) by other equipment carried. For example, a shield improves its bearer's armour value by 1, meaning that a model wearing light armour and carrying a shield would have an armour value of 5+.
Note that a model that wears no armour should be considered to have an armour value of 7+ for the purposes of Armour Save roll modifiers and any rules that improve armour value.