In an email, "CC" means "carbon copy." It indicates one or more recipients of an email in addition to the direct recipients listed in the "To" line.
The origin of the term "CC" is the carbon copy paper that was used to create copies of typewritten correspondence before the widespread use of photocopiers. The paper allowed the writer of a letter to create copies of the correspondence for additional recipients.
If the individual sending an email does not want all of the recipients to see some or all of the other recipients, the BCC line should be used for those email addresses the writer does not wish to reveal or disclose. As in the case of the CC, all of the BCC email addresses receive a copy of the email, but the list of BCC email addresses does not appear on the email that the recipients view.