They have a pretty readable explanation at the site - it goes like this
1. 2. You are in Austria and you want to visit the Presidential Palace
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Someone pastes the machine readable tag near the building, you read it off the page using your cell phone and voila, you get the URL on your cell phone's browser....
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come to think of it, would'nt it be better just to have a tinyurl link to the wikipedia article on this piece of paper?
link