This is the internet address that this filter is designed to block.
You will need to spell it the same as the browser does. e.g. if you want to block a URL http://www.foo.com/annoying/picture.gif, then you should enter www.foo.com in the host box. You should not enter foo.com or 188.8.131.52, even if they represent the same site. The case doesn't matter.
You can paste a complete URL into here, and it will be broken up into the host and the exact or partial path.