Humans were made to deal with the stress of the hunt, fight or flight.
When our bodies become stressed, the physiological reaction of our bodies in the first stage is to increased heart rate, constriction of blood vessels for non-essential parts of the body (like skin and digestion), dilation of blood vessels of heart, lungs, brain, and skeletal. Contraction of spleen, metabolize nutrients in liver to increase the availability of energy, sweating, dilation of airways, decrease in digestive activities, water retention, and elevated blood pressure.
The second stage is release of hormones which is supposed to help the body continue fighting a stressor long after the first response dissipates.
The resources of the body may eventually become so depleted that exhaustion ensues.
If stress is extreme, unusual, or long lasting, the normal mechanisms of your body to countreract stress may not be enough.
Prolonged exposure to high levels of hormones involved in stress reactions causes wasting of muscle, suppression of immune system, ulceration of the gastrointestinal tract, and failure of pancreatic beta cells. In addition pathological changes may occur because reactions may persist after the stressor has been removed.
You're probably sorry you asked now, lol.
What it means is that we havent' adapted well to the modern day stressors in our lives and our physiological reactions can be pretty harmful. Since we can't live a life that's stress free, a way to combat stress is to do something physical like exercising which helps rid the body of the hormones and uses the physiological changes to our advantage.