A pulse oximeter is a tiny clip which fits over a finger. It measures the amount of oxygen saturation (SO2) in the blood. It can subtly identify altitude sickness in the body by measuring whether there is a lack of oxygen in the body.
This is very important when you are climbing at high altitudes. It helps guides to identify whether you are safe when climbing the mountain. It can help to identify early clues to severe acute mountain sickness (AMS).