A subdomain is a subdivision of a domain name which allow you to put content in your URL before your namespace.
For example, shop.companyname.com would be a subdomain of the domain name companyname.com.
For example: If a customer buys a domain with Narrowcloud Solutions, e.g. yourdomain.com.my, they can set up subdomain, e.g. product.yourdomain.com.my or site.yourdomain.com.my.
This is an excellent way of breaking up the website if you have different regions, products or even languages.