A site's slash (often referred to as that site's "root") is the content accessed by the relative URL "/" (i.e, one forward slash). The URL is relative to your base domain.
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)