/ is where the anchor link is going to take the user that clicks on the link.
/ will take the user to the domains root, or homepage typically. In other words, if your domain was example.com, the link would take you to whatever page displays at http://example.com/
The advantage to using / is that the anchor links using it will still work if you move your websites files under a different domain name. Plus it is shorter than typing out your domain name every time.