BCC is an acronym for Blind Carbon Copy (or occasionally, Blind Courtesy Copy).
A “carbon copy” is a technique used back in the days of typewriters, where two or more copies of a typewritten page were created as the original was being typed by placing additional sheets of paper separated by carbon paper behind the original. Typing on the frontmost page would press through and cause some of the carbon from the carbon paper to be pressed onto the second copy.
Today, a carbon copy, or CC, simply refers to sending someone a copy of a communication, typically email.
A blind carbon copy simply means that the recipient of the copy is hidden from other recipients. Mail sent To: “A”, CC’ed to “B” and BCC’ed to “C” would go to A, B and C, but only A and B would be seen on the message as being recipients; C would be hidden.