Work stacks let you reassign multiple opportunities in bulk.

You will be asked to confirm your action. You will then be presented with some options.

Reassign Quotations
Tick this option if you want to reassign linked quotations to the new user.

Reassign Incomplete Tasks
This option is ticked as default. The assumption is that all open tasks linked to the opportunity should be reassigned to the new user too. This will include, phone calls, appointments, reminders and check list tasks.

Delegate Parent Record Access
This option is ticked as default to ensure that the new user has access to the record. If users have the View only my records role, they will not be able to access the lead or customer without delegated access.

For more information, check this article on delegation.