A domain name is a unique name that identifies a website. Let's dive into it that why domain names were introduced.
Every computer on the internet is identified by its IP Address assigned to it. As you know that IP Address is a series of numbers. Now let's say that I have to visit google site and I write IP Address instead of domain name. It will be difficult for me to remember the IP Address, whenever I will be accessing that site. Let's say I remember that IP Address but there are many other sites that I visit too, I am sure you cannot remember each and every IP Address. So, for human easy remembering, the domain names were introduced.
You need to understand TLDs (Top Level Domains) in order to fully grasp the concept of domain name system.