The URL

The initials stand for either: Uniform Resource Locator

or - depending upon which books you read: Universal Resource Locator.

Basically, it is the actual address of your Web Site, for example:

www.yoursite.co.uk...or with the extension

www.yoursite.com...or:

www.yoursite.org.uk... plus many others.

Very often you can actually designate your own extension, though costs may vary.  These days many seem to prefer the dot-com extension.