This refers to the entity registering your domain. When you register your domain, you will go through the process of registering your domain name / URL to associate it with your IP address. Doing so ensures that you own your site and are the only one able to access your domain. The actual domain registrar is an entity that has the access and accreditation to register domain names, including the extension, such net .com, .org, .co.za, info, etc.
