Basically when you type in an Internet address such as www.theanswerbank.co.uk, your PC or laptop sends a request off to the Internet to resolve the name (www.theanswerbank.co.uk) to an IP address eg. 192.169.1.10.
This is know as the Domain Name Service - a table of website names linked to IP addresses.
If you get that message then your Sky Internet DNS service may be not working, your laptop could be sending the request to the wrong place, or the website you are looking for doesn't exist.