Adrenaline, also called epinephrine, is a hormone released after the fight or flight response (stress response) is triggered. Adrenaline is released into the bloodstream from the adrenal glands. Adrenaline then travels to various systems, where it affects the targeted system.
In the heart, adrenaline increases the heart rate. In the lungs, adrenaline opens up the air sacs (bronchioles). In the skeletal muscles, it increases contraction. In the blood vessels, it increases constriction in most, and increases vasodilation in others. In the body, adrenaline increases blood glucose and fatty acids, giving the body the fuel it needs mount a stress response.
So all of these effects of adrenaline serve to make the body faster and stronger.