What does it mean if oxygen saturation level is low in a lung cancer patient?

Several. Pneumonia and shunting. Fluid in chest. Air leak and pneumo. Diffuse metastasis. Frequently also superimposed on emphysema from smoking.
It depends on the de. It means there is less than normal oxygen level in the blood. If the lungs are diseased, oxygen transfer from the breathed air to the blood is impaired. Normally o2 saturation is between 95% to 100%. Anything lower than 90% saturation requires oxygen supplementation.