A primary meaning of stress is the level of importance of something. Physically, stress means the level of pressure or force exerted on something. In phonetics, it means the volume at which certain words or phrases are spoken.
In physics, stress is the amount of force or strain placed on something. In physiology, stress is the body's response to a trigger or stress-inducer. Fear or a stimulating event could cause a person to experience a response of stress. Stress is also used as a verb to replace the term "worry." For instance, a person might say, "He shouldn't stress about the interview coming up next week."