Back in the days before photocopy machines were invented, duplicate copies were made by inserting a piece of carbon paper between the first and second (or maybe more) pages put into the typewriter. Whatever showed on the first page would (if you hit the keys hard enough) show on the second page.
So, CC was short for "carbon copy." I guess it is still used that way now, despite vast technological advances.
If I'm the author of a letter to you, and want you to know that I'm also sending it to another person, I will put "cc: John Jones," at the end of the letter. If I don't care to let you know about it, I won't put that. But on the copy I send to Jones I will put the letters "bcc" for "blind carbon copy," meaning "he doesn't know I sent you this copy."
That's my understanding, but it may be completely obsolete now.