Units can be deceptively tricky, depending on the unit convention being used. In fact, despite being in research to study all sorts of physical phenomena, I find one of the trickiest things to calculate are actual quantities for materials properties that make physical sense. So an excellent question, indeed!

You may be familiar with the SI unit system (e.g., meters, seconds, kilograms). There are other conventions like CGS units that defines quantities in terms of centimeters, seconds, and grams. Planck units are yet another kind of unit convention. In physics, you will come across quantities that are universal constants. These are the gravitational constant G, Coulomb constant K_c , Boltzmann constant K_B , speed of light c, and reduced Planck constant ℏ.

Each of these constants are associated with fundamental theories in physics like thermodynamics, electromagnetism, and quantum mechanics.

For example, the reduced Planck constant is a quantity that appears everywhere in quantum mechanics and can be considered a measure for the scale at which quantum mechanical phenomena begin to be observed. It turns out that you can group these five universal constants in various ways to get the Planck time, Planck length, and Planck energy, which frequently occur in many physics equations.

By definition, these five universal constants are set to 1. Part of the reason for this convention is convenience (you can leave them out in derivations) and for ease of calculations (you can do unit conversion to whatever convention you need at the very end).

Other interesting tidbit about units: what is considered a kilogram or second was not always well defined. In fact, a definition for what physical thing constitutes exactly a kilogram is is still a major challenge today. You can read more about what physical metrics are used to standardize the basic SI units and their histories here .