When you register a domain name – Domain Locking is a free security enhancement offered by most good domain name registrars to prevent unauthorized transfers of your domain to another registrar or web host.
When your domain is locked, changes cannot be made to the contact details and the nameservers. You are protected from unauthorized third parties who might try to misdirect your name servers or attempt domain name transfer without your permission. You will also be unable to inadvertently make changes to your name servers that could prevent domain hosting problems.
When you need to transfer the domain or details, you can unlock the domain name easily from your control panel anytime and change the DNS etc. When locking is enabled for a domain, any requests to transfer the domain to another registrar or hosting company will automatically fail. Only the domain owner can enable / disable locking.
Though some registrars charge for this feature, my domain name registrar GoDaddy offers free domain locking and locks the domain names by default. When you look out for a cheap domain name or low cost domain registration, do not compromise on the ability to lock your domain name. Keep your domain name secure.