What are systolic blood pressure and diastolic blood pressure?

See below. Systolic is the pressure in the arteries when the heart contracts. Diastolic is the pressure when the heart relaxes.
120/80. The upper number is the systolic and is the pressure generated when the heart contracts. The lower number (it's not a fraction, just expressed with a diagonal line between them) is the diastolic and represents residual pressure in the arterial tree when the heart is resting/filling. Both are important for health. Normal is <135/<85.