DNS (Domain Name of System).
Before 1983 or when the DNS was created, each computer or server connected to the Internet, it was only accessible by typing IP address (example 192.168.21.1), and then to connect to a website before it was necessary to know the exact IP address.
To give you an ex, to connect to Giardiniblog, instead of writing in your browser
http://www.giardiniblog.com, before you needed to write 188.121.50.96 which is IP address of this site.
So you can do this even today (in fact if you want to try writing it in your browser GiardiniBlog arrive anyway), but it is not mandatory cause the DNS server now can facilitate navigation, translating an address in ip automatically without us noticing it and without see anything.
You can understand that it is much easier to remember a name instead of a number, and DNS servers are just that, to translate the name of a site in its respect ip address.
The DNS is the service that maps an IP address to the domain of the site, here is a list of DNS server addresses used to make you understand what are:
Google DNS
8.8.8.8
8.8.4.4