A boiler is a closed vessel in which water or other liquid is heated. The heated fluid will vaporize and exit the boiler for various application processes, including power generation, heating, cooking, cleaning, etc.
About coffee, a Boiler is often related to an Espresso machine. All espresso machines have a boiler to heat the water. Most commercial espresso machines will have a double boiler system; one boiler to heat the coffee and one to heat the steam. Espresso machine produced by steam. Therefore, a single boiler will be responsible for heating the coffee extract water and one boiler in charge of steaming milk, preparing, or taking hot water.
Coffee boilers are usually made of metal that conducts heat well; the most common metals are aluminum, brass, copper, and stainless steel. Aluminum and brass are commonly used in cheaper espresso machines, while copper and stainless steel are found in more expensive, commercial ones.
Single-boiler espresso machines will often not be powerful enough to produce high-quality micro-foam, and lack stability during continuous brewing.