Your father is on oxygen therapy for, most likely, lung or heart ailments. The doctor monitored his blood oxygen levels to see how far they fell during the night. Apparently he needed more oxygen than he could obtain from the atmosphere and so an oxygen concentrator was prescribed.
Here is a good website regarding oxygen therapy from the American Lung Association:
I have no idea if your father has apnea. The test for apnea requires much more than a pulse-ox reading.
My mother-in-law and my grandfather were both on oxygen for the several years. I truly wish you and your father the best.