When you send an email there are three options that you can add some recipients to any of these fields

To

Where the main recipient(s) receive the mail.

CC

CC stands for Carbon Copy sends a copy of the email to the recipients in this field.

The recipients in this field usually just observes what is going on between you The sender and the ones who receive this mail in the To recipients field.

Both recipients in To and CC can see who received the mail but can not see who is BCC.

BCC

BCC stands for Blind Carbon Copy sends a copy of the email to the recipients in this field which they can see the email recipients in To and CC fields but the latter ones can not see the recipients in the BCC

Example

You Send Mail:To: Ali and add his supervisor CC John to confirm that the mail is sent while the general manager BCC Hasan is observing secretly without John and Ali knows.

So ALI Knows that John has received the mail but does not know about Hasan

Same applies to john that he knows the mail is sent to Ali But does not know Hasan has seen the mail.

Hasan receives the mail as well and knows that Ali is the one who is receiving the mail and John is added as well in the CC field.