What is SpO2 ?
SpO2 Is a measurement of the amount of oxygen attached to the haemoglobin cell in the circulatory system.  Put simpler it is the amount of oxygen being carried by the red blood cell in the blood.  SpO2 Is given in as a percentage, normal is around 96%.  The "S" stands for saturation.
Very simply:  SpO2 Goes up and down according to how well a person is respiring (breathing) and how well the blood is being pumped around the body, however, like most things in physiology it is more complicated than this.  
This information is provided as an aid to understanding medical lingo and in no way replaces proper training.