A DNS server or a Domain Name System is a mapping from an easy to remember domain name to the physical location of the web server that is hosting the website of that domain. The physical location is what we call the IP address. So each DNS server basically maintains a mapping table something similar to the one shown below.
When you type facebook.com in your browser’s address bar, the browser will first contact the DNS server to see where facebook.com is hosted. The DNS server will then return the IP address of that server. And the browser will then contact the server at that IP address, and then return you whatever page the server at that IP address gives it.
Domain name is like your friend’s name. IP address is like his Phone number
So domain names like google.com are websites made easy for humans to remember. It is like the difference between remembering your friend’s name and remembering his phone number. Of course, the former is easy.
Similarly, remembering the IP address of your favorite websites is like remembering numbers like 12.34.535.55, which is very difficult. And imagine having to remember numbers like these for all your favorite websites like google, facebook, amazon, twitter, etc.
So DNS servers make things easy for us by remembering the IP address against easy to remember names, and we humans have only to remember the easy names. Without the DNS servers, your browsers will never be able to find where exactly the website you are looking for is. DNS servers are the backbone of the internet. And they make things easy for us.