DNS is basically what tells your browser what address you should be going to when you point to a website. All ISPs have DNS servers which are like a "Yellow Pages" to convert the webpage address into a number that can be read by say the browser. Without DNS then "the web breaks".
Open DNS is a free DNS server that anyone can use instead of their own ISPs, all you need to do is set your PC to use OpenDNS rather than the your ISPs which is picked up by default. Theres a lot of people I know who use OpenDNS as a backup DNS even if they dont have probs with their own ISPs.
------
If you go to
https://www.opendns.com/start it tells you step by step what to do to change your DNS.
Click on the PC which opens another page - then click your Operating system (eg XP/Vista etc) and it opens another page with the details to put into your PC.