DNS stands for Domain Name System. Let's not delve deep in understanding the whole function of a DNS. For now, just remember this. It is a system that names all the computers connected to the internet in an hierarchical order. It is also a system to convert human-friendly addresses(domain names) like www.something.com to its computer-friendly address like 126.96.36.199. The DNS system is like a phone-book to all the devices connected to Internet. Whenever you try to connect to a website with its domain name, your computer performs a DNS look up. Without a DNS service, you will have to enter IP addresses instead of easy-to-remember Domain names. It is also the condition when your DNS server is down.
So now that you have a clear picture of what a DNS is, let me introduce you to Google Public DNS. It is yet again, a Google-brand. Google Provides you a DNS service to resolve the domain names. You need to configure your network settings to use the IP addresses 188.8.131.52 or 184.108.40.206 as your DNS Servers. Instructions to configure are given in the link -
If you're wondering what's the use of Google DNS, then it is the reliability and speed. Google DNS servers are always ON. I happened to switch to Google DNS only because my default DNS was down. It has never gone down since then. It also improves the security of browsing. It's Google after all.
Happy Surfing. :)