It’s the code that uniquely identifies a voucher. It is not sent to you as a text message but it is available on the 'My deals' section under your wallet in the app.
To view a voucher’s code, you must first pick a deal on the app. After picking the deal, go to My Deals tab. Click on the particular voucher you want to view its code. Below the voucher that you picked there’s a script written ‘Show code to redeem voucher’. You will see the code shown alongside it. E.g.QCR56HJ4