It’s a way to try and quantify. Mass itself is made up concept.
Rest mass is dynamic subatomic particles with their kinetic energy restrained into fields like strong /weak nuclear force, gluons, and so on.
For example, when you split the atom, the total mass of the pieces left behind is less than the mass of the atom. The energy binding things together is mass as well.
If you look at cern, they can only accelerate a particle to a particular fraction of the speed of light, after that it just gains mass as you pump energy into it.
A mechanical clock running from energy stored in its spring will weigh more than same non running clock with no energy in the spring.
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