In hubsell bounced contacts mean: "Prospects which did not receive an email due to their email being invalid."
If you want to check your bounced contacts, please take these steps:
go to contacts
click on status > bounced > apply filters
All the prospects which are under the status bounced will appear in your contacts page as shown in the gif below:
Please note: Bounced contacts are refunded automatically as long as the bounce occurs within 3 months after first receiving the data. After the three months period the bounce is not refunded. Also a bounce is not refunded if the bounce occurs in the second or later campaigns in which you target the contact.