To "copy" someone on an email message means to send it to them, even though they are not the most important recipient of the message.
In an email program, you usually write the address of the people you mainly want to send the message to in the "To:" field. If there are other people who might be interested in this information but don't have to reply to your message, you include them in the "Cc:" field. You say that you "copied them on" the message.
You can also use the shorter form of this phrase, which is just "copy (someone)":
Can you copy me and Eddie when you send that?