A hot box is an improvised appliance

Major appliance

A major appliance, or domestic appliance, is usually defined as a large machine which accomplishes some routine housekeeping task, which includes purposes such as cooking, or food preservation, whether in a household, institutional, commercial or industrial setting...

 to heat up food, usually with at least two normal incandescent light bulb

Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

s as the heat source, that is frequently found in break rooms on construction

Construction

In the fields of architecture and civil engineering, construction is a process that consists of the building or assembling of infrastructure. Far from being a single activity, large scale construction is a feat of human multitasking...

 sites. The enclosure can be made of wood, metal, or any available material that can enclose the heat. It's especially useful for heating up food in a metal container that could not otherwise be heated in a microwave oven

Microwave oven

A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

without decanting the contents. Its presence also means that a large rush of people to use any available microwave ovens is tempered by those who are able to have had their meals heated up via this different method.