Especially given it's a homework question, you're doing it the wrong way round... A better way to do this would be to look for the information, then say "I don't understand X" (ie, I can't understand exactly what the CC variable is/does). This is better for lots of reasons:
I) People are normally happier to help people if they think the person's made some effort in solving the problem themselves - this could either "If they can't be bothered then neither can I" or "I think they'll learn more if they discover it for themselves"
II) Your question is rather wide - the subject of makefiles is rather big, and even if we're only talking about using them for compiling C++ programmes, then there are still lots of things you can do with it. Providing a specific enquiry can be helpful just to know where to start and for getting answers that target the question
you want answered, rather than what the poster thinks you want answered.
III) You will have learnt a lot more, both in the way of skills and information that lasts, by reading and searching for it for yourself than being spoon-fed by members
That said, try typing "Introduction to makefiles" into Google - the first two hits are from the GNU project "make" documentation, and explain all questions I could think that you would want to ask about the subject...