Most of the air people usually breathe is nitrogen, and the composition of oxygen only accounts for more than 20%.

Generally speaking, oxygen is given first and then nitrogen is given during general anesthesia, that is, high-concentration oxygen is used to replace the air in the human respiratory tract, so that the gas in the respiratory tract is close to pure oxygen, and it is more tolerant to short-term hypoxia during anesthesia.

That's probably what it means.