A reaction roll is made in response to an attack or a hazard, representing a character's attempt to avoid or withstand an imminent effect.
Reaction rolls work like action rolls, except they don't generate Hope or Fear, don't trigger additional GM moves, and other characters can't aid you with Help an Ally.
If you critically succeed on a reaction roll, you don't clear a Stress or gain a Hope, but you do ignore any effects that would have impacted you on a success, such as taking damage or marking Stress.