DNS is an internet service which purpose is to connect domain names to the correct servers and their IP addresses. If a server has the IP address 126.96.36.199, your domain can be configured to point to this IP address in DNS.
Example: Our incoming mail server, mail.uniweb.se, is pointing to the IP address/server 188.8.131.52. It’s much easier to remember a server name, such as mail.uniweb.se, than an IP address. This is why the DNS system was made.
DNS is like a big database spread through a lot of servers on the Internet, called name servers.