Answer:
Planck began his work by trying to understand the nature of black-body radiation; ie, that of objects glowing from heat. His initial attempt failed to properly explain the spectrum of this radiation, so (by his own admission, out of desperation) he developed a hypothesis that light energy from black-bodies could only be emitted in discrete "chunks" he called "quanta," and that the energy of each such quanta was proportional to the frequency of the light. He didn't really think these quanta actually EXISTED, he just noted the mathematical fact that this assumpion led to a perfect prediction of black-body specta. It took ten years and a LOT of prodding by Albert Einstein for Planck to accept the existence of photons as more than mathematical curiosities.
Ironically, both Planck and Einstein went to their graves without accepting the quantum theories of later physicists -- theories that never would have happened without their work.