The phrase "When in Rome, do as the Romans do" is a proverb that means it is wise to follow the traditions or customs of the place you are visiting. It encourages adapting to local customs when you are in a foreign place or unfamiliar situation. The origin of this saying is attributed to Saint Ambrose in the 4th century. It started from his advice about observing Saturday as a fast day in Rome, although it was not observed that way in Milan, where he was bishop. Saint Ambrose advised that when in Rome, he would fast on Saturday as the Romans did, and when in Milan, he would follow the Milanese custom. This practical advice came to be summarized as "When in Rome, do as the Romans do" to mean that when you are in a different place, you should live according to its customs to avoid causing offense or scandal.
The phrase has since been used broadly to suggest adapting to the behavior and practices of the people around you in any new or unfamiliar situation, not just travel abroad.
In summary, "When in Rome, do as the Romans do" means to follow local customs and behave as the locals do when you are in a particular place or situation.
