When people use the expression "when in rome" what exactly does that mean?

rb6point...

The complete phrase is a bit more self-explanatory:
"When in Rome, do as the Romans do."

Examples of usage, from the GoEnglish website:

Example: "I can't eat that." Reply: "Oh, give it a try.
When in Rome, do as the Romans do."

Example: "Are you sure we can eat this with our hands?"
Reply: "Why not? All of these people are. When in Rome,
do as the Romans do."

Example: "Back home, we never sing in front of other
people." Reply: "Oh, come on. Give it a try! When in Rome,
do as the Romans do."

More on the page:
http://www.goenglish.com/WhenInRomeDoAsTheRomansDo.asp


Of course, this is where you'll here an internal tape of
your mother's voice saying, "So if all of your friends
jumped off a bridge, I suppose you'd do the same thing!?"

But then you could just answer her with another idiom,
and remind her that "50 million frenchmen can't be wrong".


And naturally, you don't really have to be in Rome for 
the saying to apply. It's generally wise to behave as
the natives do, no matter where you are, especially 
since some behaviors or mannerisms you take for granted
as being acceptable, or at least harmless, may actually
be insulting in other countries.

When I was visiting Sardinia, for example, and found 
it convenient to poor a bottle of wine by holding it
with my palm up, I was promptly corrected by a native
at my table, who indicated that pouring in this manner
was an insult, and that I should pour only with the 
palm facing down.


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Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.

Searches done, via Google:

"when in Rome, do as the Romans do"
://www.google.com/search?q=%22when+in+Rome%2C+do+as+the+Romans+do%22

here is a pretty authoritative sounding site for the the origin of the expression:
http://www.phrases.org.uk/bulletin_board/6/messages/571.html

It is also an Italian proverb, as shown on one of the sites Sublime's
last link found.  It might be pointed out that Rome  - under Vatican
rule well into the 19th  century -  was for Italians perhaps as
special and different as it was for non-Italians, but not because it
was a very pious place, on the contrary in the Renaissance.