A registrar for websites is referred to as an organization or a company that has the license and the authority to manage registrations of domain names. Such businesses are accredited by organizations like ICANN (Internet Corporation for Assigned Names and Numbers).
Registrars allow businesses to register, transfer, discontinue, and manage their domain names. Their role is important ad it helps the businesses create a unique address on the web. A couple more services, like email hosting, domain privacy, DNS management, and more, are also taken care of by the registrars.