A pulse oximeter is a small, non-invasive device, like this one, that measures the percentage of oxygen (or O2) in the blood affixed to the hemoglobin cells within the circulatory system. SPO2 is a reading delivered by the pulse oximeter to indicate how efficiently blood is transporting oxygen throughout the body. SPO2 translates to S for Saturation, P for Pulse (most pulse oximeters also measure your heart rate) and O2 means oxygen. 

Not a bad device to have in your home to determine if you or someone you love is breathing well, as respiratory illnesses like COVID-19 start to impact our ability to process oxygen, and is one of the reasons why patients in duress are often first treated with pure oxygen.