Many of Apple's machines feature ethernet ports that auto-detect if they are connected to a hub/switch/router or another computer and automatically 'cross-over' the connection if necessary, so you can connect two computers without a problem.
This just allows you to setup a network connection between two computers easily. This (nor anything else put on most laptops) allows you to use them as a Monitor/Keyboard/Mouse for a desktop however, at least not without adding software to the laptop and the desktop. But it does allow you to easily copy files between two machines without having to know very much about networking.