Carpentry is another profession learned through apprenticeship. An apprenticeship is a system of training a new generation of practitioners of a trade or profession with on-the-job training and often some accompanying study (classroom work and reading). Apprenticeship also enables practitioners to gain a license to practice in a regulated profession.
A black light is a source of electromagnetic radiation that emits near-ultraviolet light, usually in the 350-375 nanometer wavelength range. This range is just slightly below the 400nm that human eyes can detect. When this light is reflected off certain materials, it looses energy and its wavelength expands to over 400nm. The reflected light is then visible to human eyes, and the illuminated objects appear to glow.
The black wire in the box holding the light brings power to the fixture. The purpose of the switch is to interrupt that power supply when you don't want the light on. To do that, you create a switch loop, which carries the power supply from the box holding light fixture to the switch box and back up to the fixture.