A Reaction is a play performed in response to the entity being targeted by a hostile Action. Unless otherwise specified, only one Reaction may be played in response to an Action; however there is no limit to how many Reactions an entity may play in a turn. Reactions function as defenses against an enemy's attack. They may try to cause the attack to fail, reduce incoming damage, block conditions or other harmful effects, and/or attempt a counter to harm the attacking enemy.
If either the Action or Reaction lacks an opposed roll entry, both the Action and Reaction automatically succeed and take effect. Otherwise an opposed roll is performed to see if the Reaction takes effect. Defense entries on Reactions take effect even if an opposed roll is failed and the rest of the Action does not take effect.
Reactions can only be played against hostile Actions. An entity can only play a Reaction against an ally's Action if the ally is Controlled or otherwise forced to play the Action by an enemy, or if the Action has the Area Effect keyword and targets (or at least reasonably attempted to target) one or more enemies. In other words, an entity cannot “attack” themselves or an ally specifically for the purpose of playing some advantageous Reaction.
The Disoriented condition prevents an entity from playing any Reactions.
For more information, see Player's Guide Chapter 3: Scenarios.