The kilogram is defined by a lump of metal in Paris but several comparisons with identical copies showed that its mass is changing. That’s why the kilogram needs to be redefined.

Scientists want to redefine the SI units in terms of fundamental constants of nature. To redefine the kilogram scientists use the Quantum-Hall effect so mass is defined in terms of h, Planck constant.

Next constant on the list is the Ampere, the unit of electrical current, that would be redefined in terms of  the electron charge e.

More about this subject you can read in this article from Physics World