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 harmful effects, attempt a counter to harm the attacking enemy, and/or more.
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 Reaction does not take effect.
Reactions can only be played against hostile Actions. See the entry on Actions for details about restrictions on Reaction play against hostile Actions from allies (including an entity targeting itself).
For more information, see Player's Guide Chapter 3: Scenarios.