I can tell you. Because I am an expert in it as my premature son used to have that. Firstly, the stupid doctor prescribed us an oxygen tank. Oxygen tank is deadly because if that thing get near open flame, it will go kaboom. Of course the purpose is to provide oxygen for those with lungs diseases or those with difficulties breathing.
Lucky, the second doctor, a younger, very handsome doctor got me an oxygen concentrator for rental. It is a machine that takes in our room air and convert it into oxygen by filtering out the nitrogen and compressed the oxygen and it is then used for the patient.
Therefore, a portable oxygen concentrator is a smaller version and can be carried around by those folks who needs oxygen because they are unable to get enough through their regular breathing. Usually, they are patients with lungs diseases, old folks and premature babies.
Now, didn't I tell you I am an expert?