I would really appreciate it if anyone could help me understand the "normalized frequency response" of any system. I am having trouble deducing any useful information from "normalized frequency response" of a system, like this one below:

As per Nyquist theorem, the sampling frequency should be at least twice the maximum input frequency. From that perspective, the x-axis of the normalized frequency response of any system should never exceed 0.5. Why is this plot's x-axis ending at 5? Moreover, what do these bumps and ripples in the plot reveal?

Thank you.