Short answers, pulled from the story.
The kilogram is the base unit of mass in the International System of Units, with the symbol kg, and it equals one thousand grams. It is the only SI base unit whose name already contains an SI prefix, the kilo.
Since 2019 the kilogram is defined in terms of three defining constants: a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant. The Planck constant is fixed at exactly 6.62607015 when expressed in joule-seconds.
The kilogram was originally defined in 1795 during the French Revolution as the mass of one litre of water, first at 0 degrees Celsius and later at the temperature of water's maximum density, about 4 degrees Celsius. The word kilogramme was written into French law that year in the Decree of 18 Germinal.
The kilogram was redefined because the International Prototype of the Kilogram was changing, having diverged from its replicas by roughly 50 micrograms since the late 19th century. The new definition based on the Planck constant took effect on the 20th of May 2019.
The International Prototype of the Kilogram was a cylinder of platinum and iridium that served as the standard of mass for the metric system from 1889. It remained the standard for 130 years until the constant-based definition was adopted in 2019.
Because an SI unit may not carry multiple prefixes, prefixes are added to the gram rather than the kilogram, which already has the kilo prefix in its name. For example, one-millionth of a kilogram is written as one milligram, not one microkilogram.