I think it was just used to help denote concrete5 specific bits of styling, or at least classes that are output by the concrete (as opposed to ones that have been put in place by a theme developer).
I think CCM standards for 'concrete5 content management', but I could be wrong there!
If that's not the context you are asking about though you'll need to describe where you are seeing this.
When I did a search on "ccm" in Concrete5.6 local installation htdocs, it listed 69 items. Yet the same search in 5.7 yields no items.
A Google search yielded one enigmatic and brief forum conversation:
What you said about it referring to core classes as distinct from classes created by developers makes sense. Thanks.
I just felt I might be missing something that could have some significance in better understanding how C5 works.
1. In the routes – so /ccm/system/whatever/whatever. This helps ensure they're unique. We don't want to use /concrete/ in the routes because the core is actually already named "concrete/"
2. In the CSS classes. This is an aesthetic choice – it's easier to type, shorter and nicer looking than "concrete-"
As far as the acronym itself, it's historical – it's stands for "Concrete Content Management" and dates back to well before there was a concrete5. (The More You Know!™)