The redefinition of the kilogram was a part of a larger effort in the scientific community to redefine the International System of Units (SI) based on fundamental constants of nature. The previous definition of the kilogram was based on a physical artifact known as the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder stored in a vault in France.
The decision to redefine the kilogram was driven by the desire for a more stable and universal definition that does not rely on a physical object that can change over time. The new definition, which came into effect on May 20, 2019, is based on the Planck constant (h), a fundamental constant of quantum physics.
The Planck constant relates the energy of a photon to its frequency, and it has a well-defined numerical value. By fixing the value of the Planck constant, the kilogram is now defined in terms of a specific amount of energy equivalent to the mass-energy of a specific number of photons.
While photons themselves are massless particles, they do possess energy and momentum. The redefinition of the kilogram uses the energy equivalence of a certain number of photons to define its mass. The specific value you mentioned, 1.4755214 x 10^40 photons, is the number of photons whose total energy is equal to the Planck constant, h, divided by the speed of light squared, c². This energy is associated with a certain mass, and that mass is now defined as one kilogram.
In summary, the redefinition of the kilogram in terms of the number of photons is based on the energy-mass equivalence of photons rather than their actual mass. It provides a more precise and stable definition of the kilogram by connecting it to a fundamental constant of nature.