Bain Marie or hot water bath is a method commonly used in baking delicate food such as mousses, cheesecakes, custards; which should be avoided from intense and direct heat of the oven/stove. This is done by creating a gentle, evenly distributed and moist heat around the food.
Bain Marie technique requires a large shallow pan (sheet pan, roasting pan, etc.) to hold hot water and smaller pans/bowls/ramekins/dishes placed inside the large pan to hold the delicate food. It is recommended to line the large pan with a clean kitchen towel to keep the small pans from slipping/moving in the water bath. Spring form pan or the like can be used sometimes to bake mousse or cheesecake; its detachable bottom should hence be wrapped with aluminum foil to prevent water leakage. Once you fill the small pans with what you’re baking, pour hot water into the large pan about halfway up the sides of the small pans and start baking. Remember to check the water bath frequently to make sure it is at a simmer not boiling and add more hot water if necessary.