The first number (120) is the systolic blood pressure. This is the pressure reading once the heart pumps, send a large volume of blood coursing through the artery and causing there to be more pressure in the artery at that moment.
The second number (80) is the diastolic pressure. This is the pressure reading after the heart has pumped and is refilling with blood. The diastolic reading is lower than the systolic reading because there is a lower volume of blood flowing through the artery after that initial pump from the heart, and therefore there is less pressure in the artery.
***see attached pic***