A reaction roll is a roll made by the GM to determine how NPCs react to the PCs. This roll is always optional; the GM may predetermine reactions. But (at least some of the time) it's more fun to let the dice control the reactions.
To check reactions, the GM rolls 3 dice and consults the Reaction Table (below). The higher his roll, the better the NPCs will react, and the better treatment they will give the PCs.
Reaction rolls are typically made in potential combat situations, during commercial transactions, in response to requests for aid or information, and to determine the attitude and loyalty of NPC hirelings.