In the simplest terms, a website domain is a website’s name. Say, a site can be reached by typing in some numerical IP address into a browser. That’s not too convenient for visitors. So most websites get a domain. Examples of domains are google.com, facebook.com, youtube.com, etc.
The whole purpose of a domain name is to make it easier to remember the address of a website. That is why you should choose a short and memorable word or phrase.