Each web site on the internet has a numeric address that functions like coordinates on a map. Instead of pointing to a geographic location on earth, these numeric addresses, called IP addresses, point to a geographical location on the Internet.
Computers have no problems with locating and remembering numeric addresses.
In contrast, most humans have trouble remembering long complicated sequences of numbers. So, to make surfing the web easier, the domain name system was invented. This system allows people to use easy to remember names for web sites instead of those number sequences. So, in short, a domain name is nothing more than an alias for a numeric web address. It's really that simple.