The Planck theory brings together an understanding of classical physics and the statistical averaging over energy levels proposed by Boltzmann. Boltzmann's theory had trouble gaining acceptance because there was no evidence for energy levels in his time. Planck's proposal to solve the ultraviolet catastrophe using an energy ladder is a rather large leap. It would have been difficult to believe had it not been such a good fit with experiment. By making a simple assumption that the energy levels are quantized and evenly spaced, the Planck theory is able to explain the shape of the thermal emission curve, the shift with temperature (Wien displacement law), and the increase in flux with the fourth power of temperature (Stefan-Boltzmann law).
In order to be consistent with the observations Planck's constant must have a value of h = 6.626 x 10-34 J-sec. This small value implies that quantization is in an effect that will only be important on the energy scale of individual atoms and molecules, but not for macroscopic objects. The implication that light is quantized means that it travels in packets, i.e. that light has the properties of a particle. This prediction immediately sparked a great deal of creative trials to use the the particle-like properties of light to explain other unexplained phenomena. Indeed, the Planck law is the first domino, but there were many other effects that were not explained by classical phystics that were set in motion by the realization the light can behave as both a wave and a particle. When light acts like a particle we give it the name "photon".
