To copy in somebody (or copy somebody in), is to send someone a copy of an email you are sending to someone else.
Examples of use:
a) Can you copy me in on your staff emails, please?
b) Would you like me to copy you in on all the official correspondence?
c) I’ll send a copy of the meeting notes to Mr Schweizer, and copy you in.
d) Have you been copied in on the office memos?
||copy in and copies in|
Image © Fletcher Prince