Sub domain is an extension of a main domain. For example itgranules.com is a main domain and blog.itgranules.com is a sub domain of itgranules.com.
You can create any sub domain domain under your main domain e.g. shop.itgranules.com, apps.itgranules.com. Each of these sub domains can perform different functions and can be hosted at different locations to perform those functions.
As an example, itgranules.com is a main domain and provides tools where users can query different DNS queries where as blog.itgranules.com sub domain is separate and provide information and education to our readers. Both of these domains have different functions and are hosted separately.
What is a Sub Domain?