When an email bounces, it means that your sender couldn't deliver it to someone's email address. A hard bounce means that it failed for a permanent reason -- something like a fake, invalid, or blocked email address. On the other hand, a soft bounce means that it failed because of a temporary issue -- a full mailbox or an unavailable service, for example. 

To keep your list healthy, make sure you're removing all hard bounces and keeping an eye on the soft bounce addresses. By not cleansing your list, you may end up hurting your other emails' deliverability.