|Pocket size pulse oximeter|
My humble answer: I believe you are referring to pulse oximetry, or spo2. Your blood has many red blood cells (RBC), and each RBC has a hemoglobin molecule. After passing through the lungs, an oxygen molecule binds with a hemoglobin molecule. So, an spo2 gives a percentage of hemoglobin molecules (in arterial blood) that are carrying an oxygen molecule. A normal percentage is 98%. However, anything over 90% is acceptable. As we age, sleep, and with various disease conditions it's normal for this percentage to decrease. Pulse oximetry is now considered the 5th vital sign after blood pressure, heart rate, respiratory rate and temperature.