What does being on 25 percent oxygen therapy mean? how serious is this for covid patient?

Oxygen: The air we breath contains 19% oxygen (02), and our bodies developed to utilize this. If there is lung disease which requires extra oxygen, it may be added via nasal cannula (typically 1-4 liters/minute) to increase to 23% or so, for higher needs, face mask or tracheostomy 02 would be needed. This means that lungs are much less effective in transferring 02 needing assist from richer mix.