Generally, in combat, combatants have a set number of attacks in each round
and take turns dealing them out, as dictated by their initiative. However,
some actions and events can cause a combatant to let their guard down and
expose themselves to a "free attack" beyond those normally allowed for. Such
attacks are called "attacks of opportunity," and are often the result of an
action taken beyond normal melee combat actions, like attempting a bash or
stun, or of attempting to fight unarmed against an armed foe.
In some cases, advanced feats allow a combatant to carry out one of these
actions, which would normally provoke an attack of opportunity, without being
subject to that disadvantage.