Combat Advantage is a rating that increases the amount of damage dealt to an enemy when you have an advantageous position, namely when you and an ally are on opposite sides of the enemy.
Combat Advantage Total Percentage is a direct multiplier to damage dealt when Advantage is active.
Note: If an enemy has an Awareness Percentage, your character's Combat Advantage damage multiplier will be reduced by the enemy's Awareness Percentage. Non-player enemies do not have any Awareness, so this is only meaningful in PvP.
Combat Advantage is most commonly obtained by standing on the opposite side of an enemy from an ally, either a summoned companion or a party or raid member.
Combat Advantage can be also granted by certain powers: