Second Life. If you have Second Life installed, clicking on the map link will automatically teleport you to that location in-world.
Why use SLurl? In addition to allowing customized control over mapping locations in Second Life, SLurl also provides a better experience for web users who don't have Second Life. Instead of getting an error when clicking on web links that begin with "secondlife://", visiting a SLurl link gives potential new users a chance to sign up.