If you aren't even sure what it means when an email bounces on one of your campaigns, an email bounce basically means we have tried to send the email to one of your subscribers but it wasn't able to get through to them.
There's lots of reasons why an email might bounce in the first place, including...
The email address doesn't exist anymore.
The recipients email inbox is full up. This is common on services like Hotmail where user account are limited in space.
The email server doesn't exist anymore. Maybe the user hasn't renewed their domain name, etc.
As you can see, some of these reasons are sort of permanent (ie. The email address doesn't exist anymore), and some of them are temporary (ie. The receipients email inbox is full up) and as such we class the temporary bounces as "Soft" and the permanent bounces as "Hard".