The Forerunner® device has a wrist-based pulse oximeter to gauge the saturation of oxygen in your blood. Knowing your oxygen saturation can help you determine how your body is adapting to exercise and stress. Your device gauges your blood oxygen level by shining light into the skin and checking how much light is absorbed. This is referred to as SpO2.
On the device, your pulse oximeter readings appear as an SpO2 percentage. On your Garmin Connect™ account, you can view additional details about your pulse oximeter readings, including trends over multiple days (Turning On Pulse Oximeter Sleep Tracking). For more information on pulse oximeter accuracy, go to garmin.com/ataccuracy.