This answer will explain what a subdomain is and what you can use them for.

A subdomain is a subdivision of a domain, such as "support.example.com" is a subdomain of "example.com".

For example:If you buy a domain, e.g. example.com, you can set up subdomains, e.g. site1.example.com or secure.example.com. This is an excellent way of breaking up the website if you have different regions, products or even languages.