A domain name is identity on the web. It is a combination of a word or phrase and tld e.g. mywebsitename123.net.
It is also the postfix of email id e.g. myname@mywebsite.org.
Domain Registration is a periodic service. e.g. for two or five years. It can also be renewed and transferred. [subject to terms]
To check availability options and register a domain, go to Domains -> Register Domain