This article applies to 123-reg Domain names
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". These can be used to seperate different areas of your site, for example, you could have your online shop on a seperate subdomain, such as store.mydomain.com. You can even use subdomains to seperate different language versions of your site, for example en.mydomain.com for the English version, and de.mydomain.com for the German version.
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.