An attack of opportunity (often abbreviated to AoO) is a chance for Geralt's opponents to attack him while he is otherwise occupied during combat. In the case of ranged attacks, this is most likely during the "power up" portion of the attack. In the case of potions, it is quite likely during the time it takes for Geralt to use a potion if he does so during the actual battle.
Specifically, on the "Hard" difficulty level, Geralt's enemies take full advantage of these opportunities to attack him, making the combat in the game more difficult. For "Easy" or "Normal" difficulty levels, they do not.