DNS stands for Domain Name Server. Basically what it does is translate a website address into an IP Address. for example, when you go to
www.google.co.uk, it doesnt look for that site. It looks for its IP Address. So a DNS server will translate this into the IP address so that the page can be displayed.
If you receive this error then It sounds like either the DNS isnt working, or it cant find a valid IP Address. You can flush your DNS records by doing the following:
Click Start, Run, type in cmd. now type:
ipconfig/flushdns
and press enter. This will flush any DNS records because the IP address may have changed for example. This should sort the issue hopefully.