"Free Attack(s)" (and "Free Strike(s)") are terms what Chronicle X uses for attacks of opportunity, meaning attacks that a Model or Alien Unit gets to make for free if it is in Close Combat with an enemy Model or Alien Unit and the enemy moves away.
A Free Attack interrupt the movement that would move the enemy away, and the other player gets to take an out-of-turn Melee attack action, and the defender rolls one less die for Defense than it normally would. Resolve damage as usual, then if the enemy survives the attack, it may proceed with its movement.