When thinking in terms of a kitchen a "boiler" could be a pot on the stove that holds water or other liquids for the purpose of boiling. However in terms of a power plant a large ship a factory or a large building a "boiler" is a huge container usually made out of iron or another heavy metal that holds liquids or gasses primarily for the purpose of providing power or heat to the structure where it resides.
A boilermaker is someone who works on the construction installation or repair of these giant metal containers. Because boilers are built to last for several decades boilermakers are often hired to maintain them by regularly inspecting valves tubes controls and other components to ensure the boiler is in working order at all times.
There are technical schools available for anyone wanting to learn this trade but many boilermakers learn about their profession through an apprenticeship. Most jobs are available in the construction and the manufacturing industries but some jobs are also available in chemical and steel andor iron plants.
Some requirements for a professional boilermaker are the ability to work outside during inclement weather and work in spaces that are not large and the ability able to use welding equipment.
According to the website Edinformtics.com boilermakers earn approximately $45000 per year.