The subdomain is a part of the main domain. A person can use a subdomain to separate certain sections from their website so they have different areas. The sub portion of the domain is located on the left side of the domain and it is separated by a dot.
In the example my.site.com; my is the subdivision of the domain site.com
But how can a subdomain can be of use to you? You can use it for creating versions of your website in different languages, or you can separate a specific portion of it there like a support section or a knowledge base section, or a blog, or whatever you feel like subdividing from your main domain.
With registering a domain name with ETNHost you have the opportunity to create an unlimited amount of subdomains within your account.