I don't know, I would assume without a sauce (but if it is part of an ingredients list why would they say that? All steak is plain until you do something to it!).
Yohf - you eat steak with cheese on????????!!!!! Actually, that does sound quite nice in a slightly disgusting sort of way...
Haven't you ever tried them? Oh, what you've been missing... They're simply delicious (or in other words yummi)... Think of melted cheese all over your steak...
It is not a part of a recipe but somekind of intro which says what you should or shouldn't eat when you are in a restaurant. To sum up, it means without a sauce and spices?
Antonia - my guess would be that they are saying that asking for "plain steak" in a fancy restaurant is unsophisticated, and possibly rather rude - it's like saying that you want something that the chef can't mess up with all his fancy sauces, etc.