Reaction
Typically used with Courage.
In short: get ready. Characters with ranks in Reaction are top of things and ready to take immediate action. This skill represents your ability to call upon your bravery and charge into danger with a clear head instead of being startled by it. 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 respond quick enough to avoid some sort of danger. The faster you need to react, the higher the DL. If the result of your Reaction check meets or exceeds the DL, you instantly snap to attention and gain enough time to act before it's too late.
For instance, if your adventuring party springs a trap and the floor collapses, the GM might ask for a Reaction check at a DL of 15. The characters who fail the check don't have time to act before they plummet into the darkness below. However, those who manage to pass the check get a chance to Dash to safety, use Gymnastics to jump, and so on. 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 may have time to snatch her up and deliver her to safety before she gets squashed.
If you fail a Reaction check, time runs out and trouble befalls you, and the GM should be creative with the consequences. For instance, if you fail the check to save an old man from an oncoming horse, you may just not reach him in time. On the other hand if you roll a Critical Failure, maybe you can manage to push him out of the way only to get trampled yourself.