The document discusses stress theory and the biopsychosocial model of stress. It summarizes that stress has three components: 1) external stressors from one's environment, 2) internal physiological and neurological responses to stress, and 3) the interaction between external and internal factors. One of the most influential theories was proposed by Hans Selye, who described the general adaptation syndrome where the body goes through alarm, resistance, and exhaustion stages in response to prolonged stress.