An M is a unit of distance that measures how far your ship travels in a minute, introduced in the 4.0 update. None of the factions could agree on what to call the galactic unit of distance. Mars wanted it called “Mars Units”, JayCo wouldn’t budge from “Monies”, so the ‘agreement’ was to just call them M’s.

Noresist created the unit M, and the name Meow was spearheaded by Pilot Shark. Currently, the name is opposed by Lancer. See the SEC/SFC debate page on the wiki for similar disagreements over names.