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.123-reg.co.uk" is a subdomain of "123-reg.co.uk" .
For example: If a customer buys a domain with 123-reg, e.g. yourdomain.co.uk, they can set up subdomains, e.g. site1.yourdomain.co.uk or secure.yourdomain.co.uk. This is an excellent way of breaking up the website if you have different regions, products or even languages.