Ferrite Beads Design
In this context, you can (sort of) think of a ferrite bead as being a frequency-variable resistor.  The higher the frequency, the higher the resistance.

A resistor by itself doesn't attenuate anything, but does with a load following it.  Something will be on the output side of the ferrite bead, and this something will have some input impedance.  The ferrite bead impedance and this input impedance together form a voltage divider.  Note that the higher the ferrite bead impedance, the more the signal is attenuated.