Letter “m” – is generally often used as a shorthand to a phrase (current) OS thread – see: (link).

Strictly related:

1. M (struct M) – is an internal C struct containing some data related to an OS thread – see: pkg/runtime/runtime.h
2. 'm' (M m) – is a thread-local variable of type struct M containing data related to the current OS thread – see also: (link), Thread-local storage
   • NOTE: This variable is of special kind extern register, and thus it must be declared in the same way in every C unit of compilation "in all modules and libraries". This is guaranteed if you #include “runtime.h”.

You can think of it as a “machine”. There are one or more machines, and the scheduler is putting one or more goroutines onto them.

• G
• Thread-local storage