Domain locking is a recent feature that some registrars have setup. A "locked" domain will prevent any changes to nameserver information and will automatically reject any registrar transfer requests. If this feature is activated for your domain with your current registrar, you will need to disable it before initiating a domain transfer.
However, unlike other registrars, we do not automatically approve transfers of unlocked domain registrations away from us. Therefore there is no need to 'lock' domains you have registered with DreamHost. Whenever we receive a transfer away request, we email the domain owner and ask them to approve it via our "Domains > Transfer" area in the Control Panel. If they don't approve it within 4 days, we automatically deny the transfer. We keep the domains unlocked so that you can transfer them away yourself if you ever wish to do so without the extra step of unlocking them with us.
Still, you can manually manage this through the Domain Registration page of the Control Panel.