Opportunity Attack

Retreating from hand-to-hand combat, or passing through the reach of a hostile without actually engaging in combat with him, gives the enemy the opportunity to attack you easily, and interrupt your retreat or movement.
It costs the opponent only 1 action point, independent of the type of weapon being used, and it is performed as a reaction, just when the character intends to retreat, during the reatreater's turn.
The action point spent is substracted from the attacker's next turn.

The attacker still needs to roll whatever is pertinent to be able to land the attack, and then needs to roll damage as usual. See Combat Rolls.

However, if the attacker manages to inflict Shaken and/or a wound to the retreater, his retreat is interrupted due to paralyzing pain, and the retreater cannot move for the remainder of his turn.
Showing your back to a guy trying to whack you with a sword tends to be unwise.