Attackers with multiple weapons have their attack skill reduced by the sum of their weapon lengths. Thus, it is much easier to fight with two daggers (Length 2) than with two swords (Length 3). The Ambidextrous ability reduces this penalty by the amount of the Ambidextrous skill. Some weapons are considered intrinsic to a unit and do not cause multiple weapon penalties for the attacker. These are called bonus weapons.
There is no icon for this ability, but you will find it if you click on Attack. It reduces the penalty for wielding two weapons by an amount equal to the Ambidextrous level. So with Ambidextrous 2 you can dual wield two daggers with no attack skill reduction, but not if you dual wield two length 6 spears.