Acharya Prashant explains the fundamental difference between reaction and response, defining reaction as a reflex action rooted in biological evolution and physical security. He describes how these instincts, which are millions of years old and designed for survival in the jungle, still reside in our DNA. While these reflexes were once necessary for physical safety, they bypass thought and consciousness, making them animalistic and uneducated. He emphasizes that reactiveness does not involve understanding but is a chemical and biological process that occurs instantaneously, much like a machine responding to a trigger.