Hi Elizabeth (Rosebud, not Rosemary here ),

Cooking with a Bain Marie just means to place the dish of food you are cooking into a larger dish of hot/boiling water before placing into the oven. I believe you can get a specific "Bain Marie" dish, but I just use a roasting tray.

It stabilizes the food, and you can also use dishes that you would not normally place into a hot oven. For example, a single serve of whatever you are baking can be cooked in an ordinary dessert dish.

Cheers,

Rosebud