A Bcc (blind carbon copy) is a copy of an email message sent to a recipient whose email address does not appear (as a recipient) in the message.
In other words, if you get a blind carbon copy email where the sender put only your email address in the Bcc field, and put their own email in the To field, you will get the email but it will not identify your address in the To field (or any other field) once it hits your email account.
The primary reason people send blind carbon copies is to mask the other recipients from the list of recipients. Using our example again, if the sender bcced multiple people (by putting their addresses in the Bcc field before sending), none of those recipients would see who else the email was sent to.
Note: Bcc is also sometimes spelled BCC (all uppercase), bcced, bcc'd, and bcc:ed.