Epinephrine is a hormone and also known as adrenaline. It occurs naturally in the body (secreted by adrenal glands) and can also be synthetically made to be used as medication.
Strong emotions, in particular fear and anger, cause epinephrine to be secreted into the bloodstream which increases heart rate, strength, blood pressure, and metabolism. This is commonly known as the Fight or Flight response - it is your body preparing yourself for activity (fighting or running away). Epinephrine can be used medically during a heart attack to stimulate the heart into working again and during a severe allergic reaction (anaphylactic shock) to help a weakened heart pump enough blood throughout the body to keep it alive.