Power Density parameter in Ultra Low Power (ULP) devices
The higher the frequency, the more the power dissipation will be. All else equal, a processor that consumes 10 W at 25 MHz will need 20 W at 50 MHz*.

The power density (I'm not familiar with this name for it, but I'll assume you have it correct) is roughly constant for a given processor, so if you have a power density of 0.4 W/MHz, you can predict how much power it will consume for different clock speeds.

It helps you to compare the relative performance of different processors; if one processor you're considering using is rated to consume 10 W at 20 MHz, and another one consumes 20 W at 50 MHz, but you only need to clock the processor at 10 MHz, you'd have to do the math yourself to find which one would consume more power at that clock speed. But if instead they were rated at 0.5 W/MHz and 0.4 W/MHz, you can clearly see that no matter the clock speed, the second processor will consume less power than the first.

*note: It's not exactly linear, but it's a good approximation. At very low frequencies especially, though, don't expect the relation to hold. Actually, don't expect your processor to work; most processors have a minimum frequency below which they won't work.