Soft bounces are instances in which the email is undeliverable, but the specific bounce message indicates that the error may be resolved before the next send. Contacts with soft bounces will still be eligible for future sends.
Pending bounces indicate that the bounce was due to a temporary issue. HubSpot will retry sending the email until the message is delivered or until 72 hours have passed. If 72 hours go by without successfully delivering the message it becomes a soft bounce. The contact will still be eligible for future sends.
You can find more information about pending, soft, and hard bounces here.