A lot of websites can be visited via different domains. For example, the Amazon website can be visited via amazon.com but also via www.amazon.com. Both addresses can be entered in the browser and you will reach the same website.

So what's the difference?

Especially in the earlier days of the internet, it was also often called the "world wide web" (or: "www"). You still hear that term quite a bit and it's this term, that explains why some websites support such a "www" prefix in the domain.

Technically, www.amazon.com is called a "subdomain" of the "root domain" amazon.com. As the operator of a website and the owner of a domain, you can register any subdomains you want to. You could also register mysite.amazon.com, if you were the owner of the amazon.com domain. And you can then configure your web server such, that your website is served, if users enter mysite.amazon.com.

That's what many (but not all) websites do with the "www" subdomain: They register it and configure it such, that it also points at the main website.

It's not required and not all websites have it. Most users also don't type "www.somewebsite.com" manually anymore - instead just the root domain is entered in many cases.