That is the abbreviation for Uniform Resource Locator. Basically, this is the address of information that can be found on the web, such as a page, an image, or a document (such as http://www.huspot.com). URLs are important for on-page SEO because search engines crawl through the text they contain when searching for keywords. If a keyword that is to be indexed is included in the URL, you get plus points from the search engines (but unfortunately not real ones).