I am probably the last person in the world to get a smart phone and need help as I am so untechy.
This question will probably give you all a giggle (feel really stupid asking) but I really have no idea. 
Anyway tonight I used my QR Droid app for the first time to "copy" a 20% discount restuarant voucher from The Harvester sent to me in an email. Email said print off voucher or copy onto phone and had the square thing to copy beneath.
I have copied the square thing onto my phone and saved it. Is it just this square thing I flash at the restaurant to get my discount or am I supposed to do something to it to convert into a "digital" voucher on my phone?