what does Planck mean when he states that energy is quantized?
if you get down to a small enough scale, instead of appearing to be a continuous wave of energy, it appears in small "bundles" that cannot be further refined.
Read about black body radiation and the photoelectric effect. Einstein's Nobel prize was for his work showing that light appears to come in small "particles". Planck went on to formalize the mathematical description.
Planck hypothesized the quantum theory to explain black-body radiation. Einstein applied it to light, and was at first roundly mocked for his efforts.