I came across this model a few years ago and you can find it explained in slightly different forms. The basics are the same:
We have a threshold or set capacity of stress that we can handle. This can and does change depending on many factors, current feelings, state of overall physical health and mental.
All types of stress from a variety of mechanisms affect us physiologically and mentally.
The better our coping strategies, the better our resilience, the better we deal with stress.
Once our threshold is reached, symptoms are expressed and experienced. These can be in many forms such as chronic pain, chronic fatigue, sleeplessness, traumatic stress response, depression, anger, sadness...