The new definition of the kg
The kilogram used to be based on a physical object that we had decided to be the standard of a kilogram. The problem was, however, that physical objects can gain or lose mass over time which is exactly what happened to the standard kilogram. The difference was tiny but it was still there and which was inconvenient for something that we use a a standard. Thus, it was redefined in order to create a precise, unchanging standard.
Today, the kilogram is based on the Planck constant, a tiny, never changing number that plays a key role in quantum physics.
In the process of redefining it, the standard kilogram changed a tiny bit. I don’t remember how much it was but you can think of it now being 0.0000000001 kg lighter/heavier. For most of us, the redefinition will not change anything as we never deal with that small weights but for someone who is dealing with something where incredibly tiny weight differences matter, they will have to recalibrate their tools according to the new standard.
It was redefined so that it doesn’t depend on a physical object.
Until this year a kilogram was defined by the weight of the [International prototype kilogram](https://en.wikipedia.org/wiki/International_prototype_kilogram). Multiple copies of it were made for reference. The problem was that the mass between the original and the copies deviated over time, and there was no way to tell if it was the mass of the original or the copies that was changing.
So the kilogram and four other base SI units [were redefined](https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units) mathematically by setting exact numerical values based on several constants: Planck constant, elementary charge, Boltzmann constant, and Avogadro constant.