It stands for Uniform Resource Locator and it simply means the address you need to type in to get to the website of your choice. Make sure you have all the right characters in the right order.
As an example, http://www.bbc.co.uk will appear here as a direct link to that website because it's typed correctly but note where the characters are and follow my format and you shouldn't see that message again.
There are probably a billion "valid URLs" you may want to post your exact issue with some details as to what you are trying to do and what is happening.