Reaction
Typically used with Courage.
In short: get ready. Reaction represents how on top of things your character is, and how ready to take immediate action. When a potentially hazardous situation arises, a Reaction roll is used to determine in what order each participant can act.
Joining Combat
When combat begins, your GM will tell you to roll Reaction. The higher the roll, the sooner you can act. The character with the highest Reaction check result gets to review every other combatant's plans and then act first, the character with the second highest acts second, and so on down the line. More information about the involvement of Reaction in combat order can be found in Chapter 10: Combat.
Acting Quickly
Reaction is also used to react quickly and avoid some sort of danger. This represents your ability to call upon your bravery and charge into danger with a clear head. The faster you need to react, the higher the DL. For instance, if your adventuring party springs a trap and poisoned darts fly from the walls, the GM might ask for a Reaction check at a DL of 15. The characters who fail the check gain the wide-eyed condition and cannot avoid the harm coming their way. Another example: your party is scaling a cliff wall and someone above you drops an item. You can try to catch it, but you need to make a Reaction check first to determine your speed to act.
You could also make a Reaction check to help someone else in danger. For instance, some scaffolding falls from a high place and it's plummeting towards a child on the street. With a high enough Reaction check, you can snatch her up and deliver her to safety.