Classical problem in physics that wasn't resolved until Max Planck introduced quantifed energy in the atom model.

The temperature of an object reflects the kinetic energy of the atoms or molecules flying/moving around (gases/liquids) or vibrating (solids). Some part of the heat energy is related to he motion/vibration of the atoms as a whole, some part of it is related to the motion of electrons in the atom. (See Thermos for more about this.) At a given temperature all these phenomena exist together. The thermal motion of electrons and the thermal motion of ions relative to each other lead, because of Maxwell equations, to electromagnetic radiation as mentioned above. This is called thermal radiation.

The electromagnetic radiation can occur at several frequencies depending on the frequency of the vibration of charges. Physicists tried to understand how the intensity of thermal radiation depends on frequency. Experimentally obtained results seemed to refuse to fit the calculations.

The theory predicted higher contribution from higher frequencies but the reality was that the intensity dropped drastically at higher frequencies.

Why is the spectral distribution of purely thermal radiation (black body radiation) not in accordance with classical radiation theory?

Planck realized that this was understandable if one made the assumption that atoms could absorb or emit electromagnetic radiation energy only in certain portions, where a portion of energy - a quantum - was proportional to the frequency. This proportionality factor is known as Planck's constant and noted by symbol h.