Can refer a sauce or a way of serving a meal.
Sauce: In refering to a sauce, it means served with Remoulade Sauce (the classic version, not the Cajun version) to which horseradish has been added.
Serving a meal (aka Russian service): In the old French way (1600s to mid-1800s), all the dishes for a particular course were put on the table in dishes, and you helped yourself, taking what interested you (if you could get to it.) Carving was done at the table, and hot dishes would frequently get cold, especially if they were out of reach on the other side of a crowded table.
In Russian service, the food is sent out of the kitchen piping hot when the time is right for it, and offered to diners by waiters, serving from a diner’s left side. Meats are sent out of the kitchen already carved on platters. This innovation (which the Larouuse Gastronomique says was popularized in Paris in the 1860s by Urbain Dubois) reduced crowding on tables, and gave the chef more control, so that the dishes he meant to be experienced hot would be.