When we talk about a domain name, in the case of the internet we are referring to a website address.
Every computer connected to the internet is identified by a unique IP address (like 18.104.22.168).
The server that your website rests on is simply a computer, with an IP address. Numbers are not meaningful to humans and as the Internet grew we needed memorable ways of identifying those computers.
A domain name is therefore a unique identifier that points to a specific IP address.