Subdomains are names that are used within your domain name to access different parts of your website. Take for example Apple.com. Apple.com is the root domain, and if you want to access the store without having to remember a long URL, you can simply navigate to store.apple.com. The store is the subdomain. Subdomains are also known as A Records in DNS. They need to be set up appropriately to be able to point to the right location within your website.
in other words
A whole new version of your site, made by adding a subdomain.