According to MedicineNet, NPO is a medical abbreviation that means nil per os, or nothing by mouth. It means that a patient isn't allowed any food or drink for medical reasons, as stated by Wikipedia.
MedicineNet explains that one of the reasons a patient may not be allowed to eat or drink anything is because they have to be put under with anesthesia for upcoming surgery. Another reason is to avoid aspiration pneumonia, according to Wikipedia. NBM is another variation of NPO that means nil by mouth. NPO is a medical abbreviation that isn't utilized in AMA Style, which is the American Medical Association's style guide.