I've seen this graph of amplitude vs quefrency and, after reading wikipedia, still couldn't figure it out,
You and me both. : ) The math eludes me, but I think I can help from an application point of view, though Paul answered perfectly above.
I use the Cepstrum plot along with the IR and to a lesser extent, the ETC in loudspeaker design. I find it indispensable like a weird type of screwdriver that fits a weird type of screw I run into frequently. Ever work with Neutrik NL4 connectors? Find that your #1 Phillips screwdriver slips out when finalizing the torque on the cable? That's because you need a #1 "Pozidriv" screwdriver instead. It'll change your life.
I advise you to stop reading now and just remember what Paul said because I'm going to get into it. : )
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Cepstrum, like the periodic impulse response (PIR or IR), like log squared, like the energy time curve (ETC) are all ways of looking at the arrival of sound over time. Each have advantages for given situations, which is why it's nice to have access to each when interpreting the time domain behavior of sound.
I could be evil and say something like "the Cepstrum is the inverse FFT of the Log Squared representation of the IR with phase preserved", but I would never stoop to that! The Cepstrum preserves phase and - importantly - removes noise from the measurement for a much more detailed view of arrival events. Once you have a handle on the IR and Log (most people leave off the word squared) responses, I think you'll find the Cepstrum response is pretty intuitive - and you'll wonder why the heck it is so hard to find in measurement software! Don't even get me started about the Nyquist plot..
Sound is the variation of air pressure above and below atmospheric pressure.
The IR is a representation of the measurement microphone's diaphragm movement in response to the sound pressure variation it was exposed to. If all goes well, the trace above zero is the positive pressure when the mic diaphragm was pushed inward and the below zero trace is when the diaphragm is pulled outward. Thus phase is preserved in its display. The IR is a mess with acoustics work and indispensable with loudspeaker design.
The Log takes the IR data, squares it (to get rid of negatives 'cause logs don't like negative numbers), then presents this data normalized to 1, meaning that all the wiggles are relative to the peak arrival instead of an absolute number of pascals or SPL (the ETC is usually normalized to 1 as well). The Log response will always be positive. It is helpful with acoustics and loudspeaker design, but the IR, ETC and Cepstrum are better, thus obviate the need for it.
Heyser's ETC is kind of like the Log response, but has some really fancy stuff going on to more accurately display the total energy of the sound arrivals over time. Phase is not preserved (technically it is, but it isn't shown without a 3D plot with a Z axis). It's indispensable in acoustics work and helpful with loudspeaker design.
This is a comparison from the Smaart v7/v8 manuals of the IR, Log and ETC
Here's the frequency domain mess, I mean response of a 1986 vintage Klipsch Heresy II loudspeaker
This is the IR time domain response of this measurement
This is the ETC time domain response of this measurement
This is the Cepstrum time domain response of this measurement
God bless you and your precious family - Langston
Extra Credit: So where did the term Cepstrum come from? Cepstrum obviously reverses the first (4) letters of Spectrum. This is because Cepstrum is the inverse spectrum of the Log spectrum. Due to the inversion, the Cepstrum frequency domain was coined Quefrency and harmonics were coined Rahmonics.