The easiest way to understand what a domain name is would be to think about it as a house. If we pretend your website is a house and you would like to make your home open to the public, you would need a site or land to park your home, the same goes for a website, you need a domain to park it. A domain name is used to advertise your business to the public, it is the address where people can go to find you online. Domain names are unique; once you have registered yours, nobody else can use it.
Domain names can be broken into several areas which are usually a “www” or “subdomain” address followed by your main address “my company” and an extension of usually “.com”, “.org” or “.co.nz” each standing for different things, such as organization or community, country and so forth. When you register a domain it often pays to purchase 1 or 2 separate domains so similar businesses can’t use the same name or a similar extension. e.g. mybusiness.co.nz and mybusiness.com. There are over 200 different domain extensions but you shouldn’t need to purchase them all. I would recommend choosing a .com and .co.nz or similar extensions.