dazabas gave a good answer, but just in general: a URL (uniform resource locator) is the path to a resource (html page, image, PDF, etc), usually on the internet.
Example: http://i.imgur.com/Tm0nlyP.jpg
The above URL is a full URL, starting with the protocol, http:, followed by the complete path.
So to use that burger picture as background, you type
background-image: url(http://i.imgur.com/Tm0nlyP.jpg);
Usually though, you can also use a so-called relative url. Meaning if the document that contains the URL is located here http://site.com/about/index.html then the location of the document is http://site.com/about/
So using
background-image: url(paper.gif);
in said index.html will make the browser try and load http://site.com/about/paper.gif