What does Dr. Tamba mean when he says, “When you are in Rome, you do as the Romans do.?” Chapter 20

Ishmael is at the UNICEF relief camp for child Soldiers. Ishmael asks the doctor where he can find rice and fish in palm oil. Dr. Tamba says that Ishmael must to what the others do in camp. He implies that Ishmael should eat what everyone else is eating.