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