What is the difference between a soft bounce and a hard bounce?

Both mean the message did not arrive; the difference is whether it might next time:

• Hard bounce - permanent. The address does not exist or the domain is gone. Remove it immediately.
• Soft bounce - temporary. Mailbox full, server down, message too large. Worth retrying.
• Repeated soft bounces become hard - most platforms suppress an address after several consecutive failures.

Sending repeatedly to hard-bounced addresses damages your sender reputation faster than almost anything else.