A transfer lock is sometimes called a registrar lock or even a domain lock. In essence, the transfer lock is a way to prevent your domain from getting stolen or deleted. Just about every domain can be protected using the transfer lock, but there a few TLDs that use a different mechanism to lock a domain. These TLDs include .co.uk, .org.uk, .info, .be, .de, .jp and .tw.
When a domain is locked using the registrar lock it is impossible to transfer it to another registrar – you need to first unlock your domain if you want to transfer it. You may find that some domain registrars will not give you the ability to unlock your domain which essentially prevents you from transferring your domain to a different domain name registrar.