They mention Plank there in the captions. Thought a Kilogram was based on Planck's constant somehow. Kinda like how the meter is based off of light.What Legend said. The kilogram had absolutely nothing to do with Planck's constant.
The kilogram was defined by a piece of metal that looks like this:
Other countries were given replicas that were the same mass.
The problem is, that the masses changed. So when they were brought back together, the masses were no longer the same.
To fix this issue, it's being based on some universal constants instead of a piece of metal, basically.