Under ITU rules (which apply in a general way to all forms of radio communication) the code letter "C" (charlie) means "yes/affirmative/the siginificance of the previous group should be ready in the positive".
Although it doesn't figure in UK legislation, the term "charlie charlie" is widely used internationally in the same way as the UK term "affirm".
See also the separate discussion of the use of "securite", another ITU expression used widely internationally but not really seen in the UK aeronautical world.