A Domain Name is comparable to an ISBN number on a book, licence plate number on a car, or a social security number. A Domain Name is a unique identifier for a website. You can choose anything you want, as long as it's not already being used, however, it's best to pick something that helps identity your business or brand. Eg. If you have a website about hairdressing, you wouldn't call it barcelonasbestburger.com.
All domain names have a .something after them. This is called a TLD (top level domain), some have .something.somthingelse, this is a CCTLD (country code top level domain). Eg. .org, .net, .com, .services, .shop are all generic TLD's .co.uk (company, UK), .co.au (company, Australia), .ac.uk (academia, UK) are all CCTLDs Some domain TLDs and CCTLDs also have restrictions on who can register them, but very few.