Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It is there to connect Thunderbolt equipped devices like external drives or hubs (though none are available yet) or other peripherals.

It is like a USB or Firewire cable, you use a USB cable to connect a USB device to a USB port and you use a Firewire cable to connect a Firewire device to a Firewire port.
 
Unless you have a device that is specifically marketed as a thunderbolt peripheral and a machine with the corresponding port, you should ignore it. Thunderbolt cables share a port connector with mini displayport, yet thunderbolt cables will not connect to mini displayport or displayport devices. The port can be used for displayport displays, but you'd need to use a mini displayport to displayport cable, not a thunderbolt one.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.