What exactly does M/monit mean?
Monit by itself is a standalone software monitoring application. For example, it can restart Apache if it stops responding.
M/Monit is a somewhat centralized control panel. You still use Monit on the individual servers, but you get an extra control panel/GUI with it.
Monit is free, M/Monit is not.
I always assumed it stands for "multiple Monit" but I couldn't find anything to support that assumption on their website or manuals.