CC lets you send a copy of the email (and correspondence) to someone else. For example, in an email addressed [To: HR], you can [CC: your manager's email] so your manager can see the correspondence. The To: recipient will be able to see your manager's email.
If you use BCC, your manager will still see the correspondence, but the recipient will not be able to see your manager's email.
Only use CC and BCC if needed. Otherwise, you can leave it blank.