A registrar lock is a special service that many registrars offer for free. With a registrar lock, your domain will be protected from all unauthorized or accidental modifications to the domain name. With a lock in place, if there is ever a domain transfer request, it will automatically be rejected. It will also be impossible for the contact details to be updated on the domain.
When set, the following actions are disallowed by the registrar:
It is worth noting that you are not able to have a registrar lock on all types of domain extension.