M stands for mass and Z stands for charge number of ions. In mass analysis, an electron is taken from molecules to create single charged ions. If two electrons are removed, double charged ions are produced. The number of electrons removed is the charge number (for positive ions). m/z represents mass divided by charge number and the horizontal axis in a mass spectrum is expressed in units of m/z. Since z is almost always 1 with GCMS, the m/z value is often considered to be the mass.
https://www.shimadzu.com/an/gcms/support/faq/mz.html

m/z

Deprecated: mass-to-charge ratio, Thomson.

Abbreviation representing the dimensionless quantity formed by dividing the ratio of the mass of an ion to the unified atomic mass unit, by its charge number (regardless of sign). The abbreviation is written in italicized lowercase letters with no spaces.

Note 1: Mass-to-charge ratio is deprecated. Mass-to-charge ratio has been used occasionally for the horizontal axis in a plot of a mass spectrum, although the quantity measured is not the ion’s mass divided by its electric charge (SI units kg C–1). However, m/z is recommended as an abbreviation to represent the dimensionless quantity that is used almost universally as the independent variable in a mass spectrum.
Note 2: The abbreviation m/z should not be used as a variable in a mathematical equation. Instead, the variables m (in kg) and q (in C) should be used to denote mass and charge.
Note 3: The equal sign should not be used to denote a m/z value, for example, use m/z 100 and not m/z = 100.
Note 4: The thomson (Th) unit is deprecated.
http://mass-spec.lsu.edu/msterms/index.php/M/z