Brian Resnick in Vox:
Starting Monday, the kilogram will be defined by the Planck constant.
The Planck constant is a concept in quantum mechanics (i.e. the study of how the tiniest components of the universe works), which describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta). Basically, you can think of the Planck constant as the smallest action an electron can take.
What’s important about the Planck constant is that it can never, ever change. And that makes it a worthy concept to anchor the definition of the kilogram to.
But first, to appreciate why the Planck constant can define the kilogram, it’s helpful to look at how the meter — the world’s standard unit of length — was redefined in terms of the speed of light as an example of why this was necessary.
The meter was originally defined as the length of a bar at the International Bureau of Weights and Measures in France. (It was then redefined to be equal to a certain wavelength of radiation.) Again, the problem with this definition was its imprecision. It was not based on unchanging properties of the universe.
Light speed, on the other hand, is unchanging. By 1983, physicists had gotten really good at measuring the speed of light. So they used it to fix the length of the meter forever, to make it permanent.
Here’s how: They redefined the meter to be equal to the distance light travels in a vacuum in 1/299,792,458 of a second. Essentially, the definition of the meter is now baked into the definition of the speed of light.
So let’s get back to the Planck constant.
To understand, let’s take a look at it. Written out, the Planck constant is 6.62607015 × 10-34 m2kg/s.
More here.