The idea is that if you wave your NFC equipped iPhone at an NFC Mac, the computer will load all your applications, settings, and data. This will let you work at the computer as if it was your own. Then when you move out of range the machine will return to its previous state.
Here's how CultofMac's source describes it:
"The Mac authenticates with the iPhone, which contains a lot of the information the computer needs, such as bookmarks, passwords and other data," said our source, who asked to remain anonymous. "The system would essentially turn any Apple computer into your own like youre actually working on your own computer. Same settings, look, bookmarks, preferences. It would all be invisible. Your iPhone would be all you needed to unlock your Mac."
"Address book would show their contacts, and the user would have full access to their information in the same manner they would if they were working from home. This same behavior extends to even showing the same desktop picture, mouse and keyboard settings, and would eventually extend to software licenses and passwords for websites such as Facebook."
"When a person walks away with their iPhone and away from the communication link with the Mac, the original settings of the Mac would be restored. All communications and storage of passwords stays on the users iPhone, leaving nothing stored on the computer. This functionality is extended to access a users home folder, which will be stored and accessed either through storage on MobileMe or by using the Back to My Mac feature to connect to the users home folder."
Apple recently hired NFC expert Benjamin Vigier as Product Manager of Mobile Commerce.