The term "CC" stands for carbon copy. In its original context, it refers to making a duplicate of a document or letter that is being sent to another person or entity. This allows all parties involved in a transaction or communication to have access to the same information, and serves as evidence of what was communicated or agreed upon.
In modern usage, "CC" is often used as an abbreviation for "carbon copy," but can also refer to any additional recipients of an email or other electronic message.