Physical meaning of rippled spectrum
Ripples in a spectrum are an indication of echoes in time.  Specifically consider how the Fourier Transform of a sinewave is two impulses in frequency. Similarly, given the reciprocity of the Fourier Transform, a sinewave in frequency (in other words, ripples in the spectrum) is two impulses in the time domain. The Fourier Transform of the frequency response is the impulse response. A system that has an impulse response consisting of two impulses at different locations in time will output the input combined with a delayed copy of the input (echo).
In practice this can be caused by multipath delays for a signal propagating between a transmitter and receiver, reflections from poor matching on a high frequency (microwave) circuit, or other equivalent effects that result in multiple copies of the signal.