Stress and pressure are two closely related concepts; unfortunately, these two are often confused with one another.
Pressure comes in many different varieties, and the most basic one is physical pressure. When you hold a cup of coffee, you apply physical pressure to keep the cup from slipping through your hand. The cup is stressed under the pressure of your hand. If it's an aluminum can instead of a coffee cup, after finishing the drink we often increase the pressure until the structure of the aluminum can cannot handle the stress anymore - we crush the can.
Pressure is the exerted force, stress is the experience of being under that force.
In our lives we encounter many different varieties of emotional, mental or social pressure, yet the experience of stress comes only from our resistance to that pressure.