The electrons in hot objects vibrates with a range of frequencies, varying from very few vibrations per second to a huge number of vibrations per second. In fact, there is no limit to how great the frequency can be.

Classical physics explained that each frequency of vibration should have the same energy. As there is no limit to the maximum frequency thant can be achieved, there is no limit to the energy of the vibrating electrons at very high frequencies.

This means that, there should be no limit to the energy of the light produced by the electrons vibrating at high frequencies.

This explanation provided by classical theory of Maxwell was wrong, which later got clarified with the quantum mechanics concept of light given by Max Planck.