Manual blood pressure cuffs need to be used with a stethoscope so that the one administring the bp can hear the flow of blood. The numbers represen the diastolic and systolic measurement of ones actual blood pressure
You pump it to about 200 and have the stethescope on the inside of the elbow on the artery and then you release the pressure slowly. As the pressure releases, you watch the numbers and you will hear what sounds like a heartbeat. The first beat is the systolic number(top). The last one you hear is the diastolic number(bottom).
Imagine that the heart is a water pump, and one that pumps water in beats, not continuously. Imagine that the vein is a hose, a water hose. The pressure in the hose oscillates between a minimum value Vmin and a maximum value Vmax with each pumping cycle. The air cuff around the "hose/vein" is inflated until it collapses the hose completely, preventing water from flowing through it. If you apply a listening device (a stethoscope) you'd hear nothing. Now you let air leak off the cuff until you start listening to the pumping. What has happened is that the Vmax of the pump was high enough to overcome the pressure of the cuff and starts water flowing through. We read the pressure of the cuff's air and that is Vmax. The hose still collapses as soon as pressure goes below Vmax. So we hear the opening and collapsing of the hose. We continue to release air from the cuff until we cannot hear the pumping again. At this point, even the low pressure Vmin is sufficient to overcome the pressure of the cuff. The hose does not collapse anymore, we hear nothing. The pressure of the cuff at this point is Vmin. Those are the low and high readings, diastolic and systolic readings. I hope that this "crude" mechanical explanation suffices.
People read blood pressure devices that you manually squeeze in millimeters of mercury (mmHg) and are given as two numbers, for instance, 110 over 70 (110/70). The top number is the systolic blood pressure reading and it corresponds to the maximum pressure applied when the heart contracts while the base number is the diastolic blood pressure reading representing the slightest pressure in the arteries when the heart is at rest.