This answer will explain what a domain registrant is and the role they play in domain registration.
A registrant is the individual or organisation which applies for a domain name from a registrar. When registration is complete, the registrant gains the right to use that domain name for the time it is registered.
When a domain name is due for renewal, the registrant will be given the opportunity to retain control of that domain name. However, if they choose not to renew the domain name, it will be removed from the registry and will become available for anyone to purchase.