At low frequencies, the black body radiation spectrum was well explained (by  the Rayleigh Jeans formula).

At higher frequencies, it wasn’t. In fact at high frequencies, the formula predicted infinite radiated energy.

The Energy Density (energy per volume per wavelength for a given temperature) is what was incorrectly predicted by the RJ formula.

Planck treated energy as discrete (instead of continuous), allowing the density to take on finite values for all frequencies.