The domain name system (DNS) is essentially a system that assigns hard-to-remember IP addresses easy-to-remember names. Before DNS, computers would have to connect to each other using IP addresses, which are long strings of numbers like 127.0.0.1.
The domain name system assigns a name to this number (domain name), which makes it easier for visitors to the site to find it again.
When someone types in a domain name, the DNS converts into a language that a machine can understand - the IP address that was assigned to the domain name.
Thus the DNS allows us to type in "www.google.com" and have Google.com's homepage appear on our browsers, rather than having to type in a string of numbers. Either would work, but one is much easier for us to remember.