Need intuition on the frequency response of a signal
The frequency response tells you how a system reacts to a sinusoidal excitation of a given frequency, in terms of amplitude and phase.

As any signal, periodic or not, can be described as a sum of sinusoids, this allows you to deduce the response to any signal, by the superposition principle.

Interesting features of the frequency response are the continuous gain, the presence of resonances and the bandwidth. A flat curve reproduces the original excitation without deformations.

You get a feeling of a frequency response when looking at how a slowing spin-drier vibrates.