What's a sub domain?

From Jimdo Help

Some providers offer the use of sub domains. The most common and well-known sub domain is www. For example, www.jimdo.com is a subdomain of jimdo.com. There are many use-cases for sub domains, e.g. when you want to offer a shop you could set up the sub domain shop.yourdomain.com. Every sub domain can be handled individually so it's more flexible than just forwarding with yourdomain.com/shop/.