"On mange la sauce marron."
Depending on context, "on" can be translated by "one", "we" or "you".
I read comments from English speakers about the fact that using "one" was a bit pedantic, so I give you clues about "we" and "you".
In this sentence, the speaker is probably speaking "collectively" for himself and people around. So, you may translate by "we".
When statements are made in public, like "on doit se laver les mains plusieurs fois par jour", you will probably pick "you" since it is kind of an address, a semi-order: "you have to wash your hands several times a day". In the latter case, "you" can be either one or several persons.
The only real issue for you is to digest that "on" is masculine singular, even if meaning a collective subject.
I've never heard the term before, but I would say it is probably another term for espagnole sauce or brown sauce:
*Edit - wrong link for brown sauce.