In e-mail, a carbon copy (abbreviated "cc," and sometimes "fcc" for "first carbon copy") is a copy of a note sent to an addressee other than the main addressee. A blind carbon copy is a copy sent to an addressee that is not visible to the main and carbon copy addressees. For example, you may have a work colleague that acts as a back-up when you're on vacation or not at work. You don't necessarily want the people you correspond with to know that you have a back-up. So, to keep your back-up informed, you always send the back-up a blind carbon copy. The fact that a blind carbon copy was sent is not apparent to the main and carbon copy recipients.
The term is borrowed from the days of the mechanical and later the electronic typewriter (circa 1879-1979) when copies of typed sheets of paper were made by inserting a special sheet of inked paper called carbon paper into the typewriter. For two copies, you would insert carbon paper (sometimes just called a "carbon") between the original being typed and each of the two sheets that would become the carbon copies. Today, the term courtesy copy is sometimes used instead.