Question

What is the meaning of Planck's constant?

Answer 1

It is a constant that quantizes the amount of energy a photon has.
\[E = hf\]
E is the energy of the photon, h is Planck's constant, f is the frequency of waves the photon emits.

Basically, the constant puts a limit on how much energy can be emitted or absorbed by photons, i.e. in energy packets, or 'quanta'.