Response Time is a measure of how quickly you can react to events. Combat is the prime use for this stat and it directly effects your defenses as your default parry, dodge, and block is equal to Response Time. It is also added to your initiative rolls at the start of combat.
Response Time is the average of Reflexes and Endurance or $(REF+END)/2$ in full. Response Time is rarely raised directly, instead one raises Initiative, parry, dodge, or block which is done through traits.