You may know what is website. You always get something called an address to navigate to a website. A specific sequence of numbers called an IP address identifies each website. This set of numerics is what your computer uses to connect to the server where data on the website lives.
Numbers are great for a computer but for people, it's easier to use words they can remember. The words used to identify a website are known as the domain or URL and are unique to each website, as the IP address. Just think of it as a mobile phone: you want to call your friend, so just click your friend's name in the contact list, and your phone dials the phone number. You are easier to remember your friend's name but harder to remember the phone number. The domains are connected in much the same way to the IP addresses.
A domain name has two parts, for instance, as you see above "sitegine.com". ".com" is called the top-level domain (TLD) and "sitegine" is called the second-level domain (SLD) as our website identity.
Subdomains are the additional parts to your main domain under its hierarchy and are created to organize and navigate to different sections of your website. For instance, our "sitegine.com" launched a shop to store some products for selling online. So we created "shop.sitegine.com" to differentiate from our main webpage.
A question is "Why do you need a subdomain?". The great answers are: