A double boiler is a two-tiered pan set. The bottom pan holds water which is simmered, letting what is put in the top pan slowly melt or slowly cook. It is often referred to a the bain marie pan, or a bain marie. When the pan is placed over a heat source, the water on the bottom begins to boil. The heat is reduced to a simmer, and the product is cooked gently in the top pan.
It is generally used to melt chocolate and confectionery coatings, and to cook egg-based sauces gently such as Hollandaise or Bearnaise or a Creme Anglaise (sweet vanilla sauce).
Complete pan set ups include both pans together, along with a lid for the top pan. Some sets enable the cook to use both independently, while other pan sets are made so that the top one is used exclusively with the bottom pan only. A double boiler insert is the top part of a double boiler that can be used in any pan.