Blocking a domain serves to prevent theft and counterfeiting of domains. When a domain is blocked, all domain transfer requests to another registrar are automatically denied, and changes to the DNS servers associated with the domain are also prevented.
To block/unlock the domain, you have to access domain management by clicking on the option Domain which you will find in the "Basic list of services" in the dashboard.
On the domain management screen you can block or unlock your domain by clicking on the option Domain blockade.