If an email "bounces", then it hasen't reached it's intended recipient. There are two main types of bounces:
This means that the e-mail is invalid or no longer exists. You should probably update it.
This means that the e-mail you are trying to send to has a full inbox, temporarily unavailable or most commonly has an out of office reply.
We will retry sending this e-mail a number of times, but if we still can't get it through a bounce will be reported back in the campaign statistics.