For instance, if you have the domain example.com, your slash would be http://example.com/.
Most people would call this the front page of a site or the home page of a site, but that is not necessarily the case. For personal sites or hosted sites, your homepage may be at some non-slash directory. For instance, one of my homepages is http://chat.carleton.ca/~sshah. However, the slash of that site is http://chat.carleton.ca/.
Also, although the URL after the domain looks like a Unix path, there is no guarantee that the slash is a parent directory of any subsite.
This concept is important for things like robots.txt (see RobotsExclusionStandard)