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.