The Domain Name System (DNS), converts domain names into IP addresses which is how computer systems connected to the internet actually communicates with one another.  When one system wishes to interact with another system on the internet they do it by using each its IP address, not their domain name.  The domain name is just a human-readable name that acts as an alias for the underlying IP address.

When you want to access a site for the first time using your browser, say https://www.mindspun.com, and your browser does a DNS query to get the IP address of that site.  The DNS responds with the IP address of http://www.mindspun.com is 104.21.78.167.

If you wish to check the IP address associated with a domain name, there are many online tools available such as this one from MxToolbox.

This explanation only covers a small part of what DNS is and does (and significantly underplays the number of possible configurations).  In practice, DNS is a worldwide distributed database that's essential in making the internet function handling an enormous number of queries.  The largest DNS Service - Google's Public DNS - handles over 400 Billion requests per day alone.