How is oxygen saturation defined, i.e., what is it? and what does a value of 85 mean?

Safe is > 90%: Oxygen saturation is the % saturation of arterial blood with oxygen. Normal for people without lung or heart disease is usually greater than 95%. 85% is too low. If you have an oxygen saturation of 855 you need to see a doctor immediately even if it means going to Emergency.

Think public buses: Oxygen saturation is the percentage of hemoglobin molecules in your blood bound to oxygen. Normally, 95-100% of your hemoglobin is bound to oxygen, regardless of the amount of oxygen dissolved in blood. Hemoglobin carries much more oxygen than "liquid blood". So most of your body's oxygen is transported by hemoglobin, much like a public transit system. Empty buses are bad, full buses are good.