A bain-marie is like a fancy bathtub for food. When you take a bath, you fill the tub with warm water to make your body feel nice and cozy. In the same way, a bain-marie is a bowl or container filled with warm water that you put other smaller bowls or containers inside. But instead of relaxing a person's body, a bain-marie is used to keep food hot or gently melt it without burning it.
Imagine that you have some chocolate that you want to melt. You could put it in a pot on the stove, but that might cause the chocolate to scorch or stick. Instead, you can use a bain-marie. You take a big bowl and fill it with hot water. Then, you put the small bowl with the chocolate on top of the big bowl. The heat from the water heats up the small bowl, which then melts the chocolate very slowly and smoothly. This is because the water around the smaller bowl keeps the temperature constant and prevents the chocolate from burning.
A bain-marie is also used to cook delicate foods like custards or sauces. These foods can't handle direct heat very well, so they need to be cooked more gently. By putting them in a bain-marie, you can make sure they cook evenly and don't curdle or split.
So, in summary, a bain-marie is like a hot tub for food, used to keep it warm or melt it gently without burning or curdling. It is a great tool for cooks who want to make sure their food is cooked perfectly.