With the domain transfer lock, you can ensure that your domain cannot be transferred out of your account by unauthorised third parties.
If a third party tries to transfer a domain that is secured with a domain transfer lock, they will receive an error message that the transfer cannot be started.
For your safety, generic domains such as top-level domains (.com, .net, .org, etc.) are protected by a transfer lock by default. Before you attempt start a domain transfer to another registrar (domain provider), you must disable the transfer lock.