Bounced emails are emails that are returned to the "sender" because the email cannot be delivered. The two most common delivery failures are (1) the user does not exist, and (2) the user's inbox is full.
Usually, by the time the receiver's mail server realizes that the email is not deliverable, the connection to the real sender's server has already been dropped. This results in the email being bounced back to the user listed in the "from" line. Spammers always put someone else's name in that field because they don't want the bounced emails coming back to themselves.