Ok. When the heart beats, it's contracting, thus increasing your blood pressure. This is how it pushes blood around the circulatory system. When it relaxes again, it reduces your blood pressure and pulls blood around your circulatory system. The first number is measured at the high point of pressure. The second is the number at its lowest point. 120 (high) over 80 (low) is usually considered ideal, iirc, but it can vary without being a problem.