A pulse oximeter is a medical device used to measure the oxygen saturation level (SpO2) in the blood and the heart rate. It's a small, portable device that typically clips onto a finger, although there are also models that can be worn on the earlobe or other parts of the body. The device works by emitting light wavelengths through the skin and measuring the amount of light absorbed by oxygenated and deoxygenated hemoglobin. Based on this measurement, it can determine the oxygen saturation level, which indicates how well oxygen is being transported to the body's extremities.