A bain marie can mean a couple of things in professional kitchens. As a piece of equipment, it is a stainless steel round cylinder typically used to hold liquids. They come in varying sizes and many steam tables have built in support for holding them for service. Cooks commonly keep tools in bain maries for quick access during service.
A bain marie can also mean a hot water bath. The literal translation from French is "water bath". For instance if a recipe says to bake something like a custard or paté en croute in a bain marie. It means to suround the baking dish in water to moderate the heat of the oven. Water temperature will not rise above 212 farenheit so it is the ideal method for delicate baking projects such as setting egg.