Prophylaxis refers to all the things people do to prevent disease. Prophylaxis is part of medicine.
If you've ever heard of someone using prophylactics to prevent a disease — often a sexually transmitted disease — then you have a clue to the meaning of prophylaxis, which refers to the prevention of disease. Anything you do to stay healthy and keep illness at bay would be categorized as prophylaxis. Antibiotic prophylaxis refers to giving patients antibiotics to prevent a bacterial infection, instead of curing an infection that already exists.