Currently, you can use the app to file your declaration at 4 airports. Miami, Atlanta, Chicago-O'hare, and Seattle. You can fill it out 24 hours in advance of your arrival in the US. Once you land at one of the airports listed above, you can send your declaration in electronically and you will be issued a barcode in the app on your smarthphone/tablet. The barcode will be valid for 4 hours. You will present your smartphone/tablet together with your passport (and others in group) at the specially marked line. Your identity will be verified and your barcode scanned and you will be sent on your way.
There is a website setup with all the details,
http://mobilepassport.us/Forgot to add this is only valid for US and Canadian passport holders.