What does blood pressure reading means?

To push a fluid mixture such as blood through tubes pressure (force) must be applied, in the case of blood the pressure is applied by the heart.

Blood pressure has two readings systolic and diastolic. Systolic pressure it the higher pressure. It actually pushes the blood through the blood vessels when the heart contracts (beats/pumps). Diastolic is the minimum pressure that happens when the heart has finished a beat and is refilling with blood for the next beat. The pressure is never zero because the vessels or tubes that carry the blood are stretched and are squeezing on the blood even when the heart is not pushing.

Both the systolic and diastolic pressures are important. If these pressures get high for a short period (unless the systolic is extremely high) it's not usually a problem. But, if the systolic remains too high for long periods it stresses the heart and the vessels and over time can cause either to fail. If the diastolic is too high it never lets the heart and vessels rest/relax which can also cause failures over time.