What is the significance of an organic compound's M peak? Background

On the topic of identifying an organic compounds using peaks generated in Mass Spectrometry, a rule of thumb expressed in the UK educational textbook, CGP, is that that "the M peak is the one with the second highest mass/charge ratio".

Question

What is the significance of this M peak anyway?
"M" stands for molecular ion.  It is often represented as $\ce{M, M^{+} or M^{+~.}}$. In the mass spec experiment, an electron is knocked out of some of the sample molecules.  The peak at M/q (q=1) represents the molecular weight of the entire molecule, less an electron, before it fragments to smaller ions.  From precise measurement of the molecular ion (e.g. 82.34567890) you can determine the exact molecular formula of your sample compound.  This can help greatly with sample identification.