\@M do in the definition of
\@makechapterhead in book class?
I took a quick look at
latex.ltx and couldn't find any explanation.
See the question \@ne \tw@ \thr@@ for more details on the reasons.
See also Documentation reference for LaTeX internal commands? for a list of further similar macros. I also can recommend you
At line 316 you find
At line 300 you find