It's the address of your website lets say and if someone wants to set up a website, they need to pay a small annual fee to register a domain name. The domain name can then be "pointed" to a website containing the web pages that the person wants to set up. When visitors type that web address into their browser, they will be brought to your website. Say like if you were to type "showbiz.com" into your browser, you will be brought to the main page of that website. Sounds more complicated than it is.