How to interpret the vertical axis of this attenuation plot?
It does not show attenuation versus frequency, it shows impedance versus frequency.

When this component is used as a two-terminal device, this is the impedance it presents across its terminals. With a driving RF voltage, you can estimate the current it will allow to flow into a short circuit (like a shunt capcitor at RF).

The great benefit of having a component that looks resistive, is that it will absorb RF energy, rather than just reflecting it.

If you build it into a filter, with this component from input to output, and a C to ground on both terminals, it will make some sort of low pass filter. If you have the plot to below 1MHz, you will be able to estimate the effective low frequency inductance from the rate of change of impedance with frequency. The resistive part of the impedance will damp the filter. As it's frequency varying, you can't design the filter as a classical Butterworth or Cheby. You can however estimate the attenuation at various spot frequencies in a simulator.